Light-emitting element, display device, electronic device, and lighting device

ABSTRACT

A light-emitting element with high luminous efficiency is provided. The light-emitting element contains a first organic compound and a second organic compound. The first and second organic compounds form an exciplex. The first organic compound emits no fluorescence but phosphorescence at a temperature ranging from low temperature to normal temperature. The luminescence quantum yield of the first organic compound is higher than or equal to 0% and lower than or equal to 40% at room temperature. Light emitted from the light-emitting element includes light emitted from an exciplex formed by the first organic compound and the second organic compound.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a light-emittingelement, a display device including the light-emitting element, anelectronic device including the light-emitting element, or a lightingdevice including the light-emitting element.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, a liquidcrystal display device, a light-emitting device, a lighting device, apower storage device, a storage device, a method for driving any ofthem, and a method for manufacturing any of them.

2. Description of the Related Art

In recent years, research and development of light-emitting elementsusing electroluminescence (EL) have been actively conducted. In thebasic structure of such a light-emitting element, a layer containing alight-emitting substance (an EL layer) is provided between a pair ofelectrodes. Voltage application between the electrodes of this elementcan cause light emission from the light-emitting substance.

Since the above light-emitting element is a self-luminous type, adisplay device using this light-emitting element has advantages such ashigh visibility, no necessity of a backlight, and low power consumption.The light-emitting element also has advantages in that, for example, itcan be manufactured to be thin and lightweight, and has high responsespeed.

In a light-emitting element (e.g., an organic EL element) whose EL layercontains a light-emitting organic compound as a light-emitting substanceand is provided between a pair of electrodes, voltage applicationbetween the pair of electrodes causes injection of electrons from acathode and holes from an anode into the EL layer having alight-emitting property; thus, current flows. By recombination of theinjected electrons and holes, the light-emitting organic compound isbrought into an excited state to provide emission.

Excited states that can be formed by an organic compound are a singletexcited state (S*) and a triplet excited state (T*). Light emission fromthe singlet excited state is referred to as fluorescence, and lightemission from the triplet excited state is referred to asphosphorescence. The formation ratio of S* to T* in a light-emittingelement is 1:3. Thus, a light-emitting element containing a compoundthat emits phosphorescence (phosphorescent compound) has higher luminousefficiency than a light-emitting element containing a compound thatemits fluorescence (fluorescent compound). For this reason,light-emitting elements containing phosphorescent compounds capable ofconverting triplet excitation energy into light emission have beenactively developed in recent years. In Non-Patent Document 1, forexample, the temperature dependence of the luminescence quantum yield ofan Ir complex, which is a phosphorescent compound, is investigated indetail to find the relationship between the molecular structure of an Ircomplex and its luminescence quantum yield and the reason thereof.

As a material capable of partly converting triplet excitation energyinto light emission, a thermally activated delayed fluorescent (TADF)material is known in addition to a phosphorescent compound. In athermally activated delayed fluorescent material, a singlet excitedstate is generated from a triplet excited state by reverse intersystemcrossing, and the singlet excited state is converted into lightemission.

Patent Document 1, for example, discloses a method in which an exciplexformed by two kinds of organic compounds is used as a thermallyactivated delayed fluorescent material because the energy differencebetween a singlet excited state and a triplet excited state is small.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2014-045184

Non-Patent Document

-   [Non-Patent Document 1] T. Sajoto and five others, Journal of    American Chemical Society, vol. 131, 9813 (2009).

SUMMARY OF THE INVENTION

To increase the luminous efficiency of a light-emitting elementcontaining a thermally activated delayed fluorescent material, efficientgeneration of a singlet excited state from a triplet excited state ispreferable. A method for efficiently generating a singlet excited statefrom a triplet excited state in a light-emitting element in which anexciplex is used as a thermally activated delayed fluorescent materialneeds to be developed to further improve the luminous efficiency of thelight-emitting element.

In view of the above, an object of one embodiment of the presentinvention is to provide a light-emitting element with high luminousefficiency. Another object of one embodiment of the present invention isto provide a light-emitting element with low driving voltage. Anotherobject of one embodiment of the present invention is to provide alight-emitting element with low power consumption. Another object of oneembodiment of the present invention is to provide a highly reliablelight-emitting element. Another object of one embodiment of the presentinvention is to provide a novel light-emitting element. Another objectof one embodiment of the present invention is to provide a novellight-emitting device. Another object of one embodiment of the presentinvention is to provide a novel display device.

Note that the description of the above objects does not preclude theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

One embodiment of the present invention is a light-emitting elementcontaining two kinds of organic compounds that form an exciplex. One ofthe organic compounds has a function of converting triplet excitationenergy into light emission.

One embodiment of the present invention is a light-emitting elementincluding a light-emitting layer. The light-emitting layer contains afirst organic compound and a second organic compound. A LUMO level ofone of the first organic compound and the second organic compound ishigher than or equal to a LUMO level of the other of the first organiccompound and the second organic compound. A HOMO level of the one of thefirst organic compound and the second organic compound is higher than orequal to a HOMO level of the other of the first organic compound and thesecond organic compound. A combination of the first organic compound andthe second organic compound is capable of forming an exciplex. The firstorganic compound is capable of converting triplet excitation energy intolight emission at a temperature higher than or equal to 77 K and lowerthan or equal to 313 K. The first organic compound has a luminescencequantum yield higher than or equal to 0% and lower than or equal to 40%at room temperature. Light emitted from the light-emitting layerincludes light emitted from the exciplex.

Another embodiment of the present invention is a light-emitting elementincluding a light-emitting layer. The light-emitting layer contains afirst organic compound and a second organic compound. A LUMO level ofone of the first organic compound and the second organic compound ishigher than or equal to a LUMO level of the other of the first organiccompound and the second organic compound. A HOMO level of the one of thefirst organic compound and the second organic compound is higher than orequal to a HOMO level of the other of the first organic compound and thesecond organic compound. A combination of the first organic compound andthe second organic compound is capable of forming an exciplex. The firstorganic compound is capable of emitting no fluorescence and emittingphosphorescence at a temperature higher than or equal to 77 K and lowerthan or equal to 313 K. The first organic compound has a luminescencequantum yield higher than or equal to 0% and lower than or equal to 40%at room temperature. Light emitted from the light-emitting layerincludes light emitted from the exciplex.

Another embodiment of the present invention is a light-emitting elementincluding a light-emitting layer. The light-emitting layer contains afirst organic compound and a second organic compound. A LUMO level ofone of the first organic compound and the second organic compound ishigher than or equal to a LUMO level of the other of the first organiccompound and the second organic compound. A HOMO level of the one of thefirst organic compound and the second organic compound is higher than orequal to a HOMO level of the other of the first organic compound and thesecond organic compound. A combination of the first organic compound andthe second organic compound is capable of forming an exciplex. The firstorganic compound contains Ru, Rh, Pd, Os, Ir, or Pt. The first organiccompound has a luminescence quantum yield higher than or equal to 0% andlower than or equal to 40% at room temperature. Light emitted from thelight-emitting layer includes light emitted from the exciplex.

In any of the above structures, a lowest triplet excitation energy levelof the first organic compound is preferably higher than or equal to alowest triplet excitation energy level of the second organic compound.

In any of the above structures, the first organic compound preferablycontains Ir. The first organic compound preferably includes a ligandcoordinated to the Ir. The ligand preferably includes anitrogen-containing five-membered heterocyclic skeleton.

In any of the above structures, the second organic compound ispreferably capable of transporting an electron. The second organiccompound preferably includes a π-electron deficient heteroaromaticskeleton.

In any of the above structures, luminous efficiency of the light emittedfrom the exciplex is preferably higher than luminous efficiency of lightemitted from the first organic compound.

Another embodiment of the present invention is a display deviceincluding the light-emitting element having any of the above structuresand at least one of a color filter and a transistor. Another embodimentof the present invention is an electronic device including the displaydevice and at least one of a housing and a touch sensor. Anotherembodiment of the present invention is a lighting device including thelight-emitting element having any of the above structures and at leastone of a housing and a touch sensor. The category of one embodiment ofthe present invention includes not only a light-emitting deviceincluding a light-emitting element but also an electronic deviceincluding a light-emitting device. Accordingly, the light-emittingdevice in this specification refers to an image display device or alight source (including a lighting device). The light-emitting devicemay include, in its category, a display module in which a connector suchas a flexible printed circuit (FPC) or a tape carrier package (TCP) isconnected to a light-emitting element, a display module in which aprinted wiring board is provided on the tip of a TCP, or a displaymodule in which an integrated circuit (IC) is directly mounted on alight-emitting element by a chip on glass (COG) method.

One embodiment of the present invention can provide a light-emittingelement with high luminous efficiency. One embodiment of the presentinvention can provide a light-emitting element with low driving voltage.One embodiment of the present invention can provide a light-emittingelement with low power consumption. One embodiment of the presentinvention can provide a highly reliable light-emitting element. Oneembodiment of the present invention can provide a novel light-emittingelement. One embodiment of the present invention can provide a novellight-emitting device. One embodiment of the present invention canprovide a novel display device.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily have all the effects described above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a light-emitting elementof one embodiment of the present invention.

FIG. 2A is a schematic cross-sectional view of a light-emitting layer ina light-emitting element of one embodiment of the present invention, andFIGS. 2B and 2C illustrate the correlations of energy levels.

FIG. 3A is a schematic cross-sectional view of a light-emitting layer ina light-emitting element of one embodiment of the present invention, andFIGS. 3B and 3C illustrate the correlations of energy levels.

FIGS. 4A and 4B each illustrate the correlation of energy levels in alight-emitting layer in a light-emitting element of one embodiment ofthe present invention.

FIG. 5 is a schematic cross-sectional view of a light-emitting elementof one embodiment of the present invention.

FIG. 6 is a schematic cross-sectional view of a light-emitting elementof one embodiment of the present invention.

FIGS. 7A and 7B are each a schematic cross-sectional view of alight-emitting element of one embodiment of the present invention.

FIGS. 8A and 8B are a top view and a schematic cross-sectional viewillustrating a display device of one embodiment of the presentinvention.

FIGS. 9A and 9B are schematic cross-sectional views each illustrating adisplay device of one embodiment of the present invention.

FIGS. 10A and 10B are schematic cross-sectional views each illustratinga display device of one embodiment of the present invention.

FIG. 11 is a perspective view illustrating a display module of oneembodiment of the present invention.

FIGS. 12A to 12G illustrate electronic devices of one embodiment of thepresent invention.

FIGS. 13A to 13C illustrate a display device of one embodiment of thepresent invention.

FIG. 14 illustrates lighting devices of one embodiment of the presentinvention.

FIG. 15 shows the luminance-current density characteristics oflight-emitting elements in Example.

FIG. 16 shows the luminance-voltage characteristics of light-emittingelements in Example.

FIG. 17 shows the current efficiency-luminance characteristics oflight-emitting elements in Example.

FIG. 18 shows the power efficiency-luminance characteristics oflight-emitting elements in Example.

FIG. 19 shows the external quantum efficiency-luminance characteristicsof light-emitting elements in Example.

FIG. 20 shows electroluminescence spectra of light-emitting elements inExample.

FIG. 21 shows emission spectra of a thin film in Example.

FIG. 22 shows emission spectra of a thin film in Example.

FIG. 23 shows an absorption spectrum and an emission spectrum of acompound in Example.

FIG. 24 shows the luminance-current density characteristics of alight-emitting element in Example.

FIG. 25 shows the luminance-voltage characteristics of a light-emittingelement in Example.

FIG. 26 shows the current efficiency-luminance characteristics of alight-emitting element in Example.

FIG. 27 shows the power efficiency-luminance characteristics of alight-emitting element in Example.

FIG. 28 shows the external quantum efficiency-luminance characteristicsof a light-emitting element in Example.

FIG. 29 shows an electroluminescence spectrum of a light-emittingelement in Example.

FIG. 30 shows an absorption spectrum of a compound in Example.

FIG. 31 shows the luminance-current density characteristics oflight-emitting elements in Example.

FIG. 32 shows the luminance-voltage characteristics of light-emittingelements in Example.

FIG. 33 shows the current efficiency-luminance characteristics oflight-emitting elements in Example.

FIG. 34 shows the power efficiency-luminance characteristics oflight-emitting elements in Example.

FIG. 35 shows the external quantum efficiency-luminance characteristicsof light-emitting elements in Example.

FIG. 36 shows electroluminescence spectra of light-emitting elements inExample.

FIG. 37 shows emission spectra of a thin film in Example.

FIG. 38 shows emission spectra of a thin film in Example.

FIG. 39 shows emission spectra of a thin film in Example.

FIG. 40 shows the luminance-current density characteristics oflight-emitting elements in Example.

FIG. 41 shows the luminance-voltage characteristics of light-emittingelements in Example.

FIG. 42 shows the current efficiency-luminance characteristics oflight-emitting elements in Example.

FIG. 43 shows the power efficiency-luminance characteristics oflight-emitting elements in Example.

FIG. 44 shows the external quantum efficiency-luminance characteristicsof light-emitting elements in Example.

FIG. 45 shows electroluminescence spectra of light-emitting elements inExample.

FIGS. 46A and 46B each show an absorption spectrum and an emissionspectrum of a compound in Example.

FIG. 47 shows the external quantum efficiency-luminance characteristicsof light-emitting elements in Example.

FIG. 48 shows electroluminescence spectra of light-emitting elements inExample.

FIG. 49 shows transient EL curves of light-emitting elements in Example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the drawings. However, the present invention is notlimited to the following description, and the mode and details can bevariously changed unless departing from the scope and spirit of thepresent invention. Accordingly, the present invention should not beinterpreted as being limited to the content of the embodiments andexamples below.

Note that the position, size, range, or the like of each componentillustrated in the drawings and the like are not accurately representedin some cases for easy understanding. Therefore, the disclosed inventionis not necessarily limited to the position, size, range, or the like asdisclosed in the drawings and the like.

Note that the ordinal numbers such as “first,” “second,” and the like inthis specification and the like are used for convenience and do notdenote the order of steps or the stacking order of layers. Therefore,for example, description can be made even when “first” is replaced with“second” or “third,” as appropriate. In addition, the ordinal numbers inthis specification and the like are not necessarily the same as thosewhich specify one embodiment of the present invention.

In the description of modes of the present invention in thisspecification and the like with reference to the drawings, the samecomponents in different diagrams are denoted by the same referencenumeral in some cases.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other. For example, the term “conductive layer”can be changed into the term “conductive film” in some cases.Furthermore, the term “insulating film” can be changed into the term“insulating layer” in some cases.

In this specification and the like, a singlet excited state (S*) refersto a singlet state having excitation energy. An S1 level refers to thelowest level of the singlet excitation energy level, that is, theexcitation energy level of the lowest singlet excited state (S1 state).A triplet excited state (T*) refers to a triplet state having excitationenergy. A T1 level refers to the lowest level of the triplet excitationenergy level, that is, the excitation energy level of the lowest tripletexcited state (T1 state). Note that in this specification and the like,simple expressions “singlet excited state” and “singlet excitationenergy level” mean the S1 state and the S1 level, respectively, in somecases. In addition, expressions “triplet excited state” and “tripletexcitation energy level” mean the T1 state and the T1 level,respectively, in some cases.

In this specification and the like, a fluorescent compound refers to acompound that emits light in a visible light region when the relaxationfrom a singlet excited state to a ground state occurs. A phosphorescentcompound refers to a compound that emits light in a visible light regionat room temperature when the relaxation from a triplet excited state toa ground state occurs. That is, a phosphorescent compound refers to acompound that can convert triplet excitation energy into visible light.

Note that “room temperature” in this specification and the like is atemperature ranging from 0° C. to 40° C.

In this specification and the like, a wavelength range of blue refers toa wavelength range which is greater than or equal to 400 nm and lessthan 490 nm, and blue light emission has at least one emission spectrumpeak in the wavelength range. A wavelength range of green refers to awavelength range which is greater than or equal to 490 nm and less than580 nm, and green light emission has at least one emission spectrum peakin the wavelength range. A wavelength range of red refers to awavelength range which is greater than or equal to 580 nm and less thanor equal to 680 nm, and red light emission has at least one emissionspectrum peak in the wavelength range.

Embodiment 1

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described below with reference to FIG. 1,FIGS. 2A to 2C, FIGS. 3A to 3C, and FIGS. 4A and 4B.

<Structure Example 1 of Light-Emitting Element>

First, the structure of the light-emitting element of one embodiment ofthe present invention will be described below with reference to FIG. 1.

FIG. 1 is a schematic cross-sectional view of a light-emitting element150 of one embodiment of the present invention.

The light-emitting element 150 includes a pair of electrodes (anelectrode 101 and an electrode 102) and an EL layer 100 between the pairof electrodes. The EL layer 100 includes at least a light-emitting layer130.

The EL layer 100 illustrated in FIG. 1 includes functional layers suchas a hole-injection layer 111, a hole-transport layer 112, anelectron-transport layer 118, and an electron-injection layer 119, inaddition to the light-emitting layer 130.

Although description in this embodiment is given assuming that theelectrode 101 and the electrode 102 of the pair of electrodes serve asan anode and a cathode, respectively, they are not limited thereto forthe structure of the light-emitting element 150. That is, the electrode101 may be a cathode, the electrode 102 may be an anode, and thestacking order of the layers between the electrodes may be reversed. Inother words, the hole-injection layer 111, the hole-transport layer 112,the light-emitting layer 130, the electron-transport layer 118, and theelectron-injection layer 119 may be stacked in this order from the anodeside.

The structure of the EL layer 100 is not limited to the structureillustrated in FIG. 1, as long as at least one of the hole-injectionlayer 111, the hole-transport layer 112, the electron-transport layer118, and the electron-injection layer 119 is included. Alternatively,the EL layer 100 may include a functional layer which has a function oflowering a hole- or electron-injection barrier, improving a hole- orelectron-transport property, inhibiting a hole- or electron-transportproperty, or suppressing quenching by an electrode, for example. Notethat the functional layer can be either a single layer or stackedlayers.

<Light Emission Mechanism 1 of Light-Emitting Element>

Next, the light emission mechanism of the light-emitting layer 130 willbe described below.

In the light-emitting element 150 of one embodiment of the presentinvention, voltage application between the pair of electrodes (theelectrodes 101 and 102) allows electrons and holes to be injected fromthe cathode and the anode, respectively, into the EL layer 100 and thuscurrent flows. By recombination of the injected electrons and holes,excitons are formed. The ratio (generation probability) of singletexcitons to triplet excitons which are generated by recombination ofcarriers (electrons and holes) is 1:3 according to the statisticallyobtained probability. In other words, the generation probability ofsinglet excitons is 25% and the generation probability of tripletexcitons is 75%. Thus, it is important to make the triplet excitonscontribute to light emission in order to improve the luminous efficiencyof the light-emitting element. For this reason, a material that has afunction of converting triplet excitation energy into light emission ispreferably used as a light-emitting material for the light-emittinglayer 130.

As the material that has a function of converting triplet excitationenergy into light emission, a compound that can emit phosphorescence(hereinafter, also referred to as a phosphorescent compound) can begiven. A phosphorescent compound in this specification and the like is acompound that emits phosphorescence and no fluorescence at a temperaturehigher than or equal to a low temperature (e.g., 77 K) and lower than orequal to room temperature (i.e., higher than or equal to 77 K and lowerthan or equal to 313 K). The phosphorescent compound preferably containsa heavy atom in order to efficiently convert triplet excitation energyinto light emission. In the case where the phosphorescent compoundcontains a heavy atom, intersystem crossing between a singlet state anda triplet state is promoted by spin-orbit interaction (interactionbetween spin angular momentum and orbital angular momentum of anelectron), and transition between a singlet ground state and a tripletexcited state of the phosphorescent compound is allowed. This means thatthe probability of transition between the singlet ground state and thetriplet excited state of the phosphorescent compound is increased; thus,the luminous efficiency and the absorption probability which relate tothe transition can be increased. Accordingly, the phosphorescentcompound preferably contains a metal element with large spin-orbitinteraction, specifically, a transition metal element. It isparticularly preferable that a platinum group element (ruthenium (Ru),rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), or platinum(Pt)), especially iridium, be contained because the probability ofdirect transition between a singlet ground state and a triplet excitedstate can be increased.

As the material that has a function of converting triplet excitationenergy into light emission, a thermally activated delayed fluorescent(TADF) material can also be given. Note that the thermally activateddelayed fluorescent material is a material having a small differencebetween the T1 level and the S1 level and having a function ofconverting triplet excitation energy into singlet excitation energy byreverse intersystem crossing. Thus, the thermally activated delayedfluorescent material can upconvert triplet excitation energy intosinglet excitation energy (i.e., reverse intersystem crossing) using alittle thermal energy and can efficiently exhibit light (fluorescence)emission from a singlet excited state. An exciplex (excited complex) hasan extremely small difference between the S1 level and the T1 level andfunctions as a thermally activated delayed fluorescent material that canconvert part of triplet excitation energy into light emission.

In one embodiment of the present invention, an exciplex formed by twokinds of materials is used as the light-emitting material for thelight-emitting layer 130. One of the two kinds of materials is acompound that has a function of converting triplet excitation energyinto light emission by itself.

The present inventors have found that an exciplex with high luminousefficiency can be formed as long as a compound that has a function ofconverting triplet excitation energy into light emission by itself isused as one of the compounds that form an exciplex even when thecompound has a low luminescence quantum yield. When a compoundcontaining a heavy atom is used as one of the compounds that form anexciplex, intersystem crossing between a singlet state and a tripletstate is promoted by spin-orbit interaction (interaction between spinangular momentum and orbital angular momentum of an electron). In otherwords, reverse intersystem crossing from a triplet excited state to asinglet excited state in an exciplex is promoted; thus, the generationprobability of singlet excited states in the exciplex can be increased.As a result, an exciplex that emits light from a singlet excited stateefficiently can be formed. The probability of transition from a tripletexcited state to a singlet ground state can also be increased; thus, anexciplex that emits light from a triplet excited state efficiently canbe formed. There is no limitation on the excited state from which anexciplex emits light (because singlet energy and triplet energy of theexciplex are close to each other). Note that the excitation (emission)lifetime of an exciplex is significantly shorter than that of a normalthermally activated delayed fluorescent material. This allowssuppression of deterioration from an excited state, leading to alight-emitting element with an essentially long driving lifetime. Toachieve this, one of the compounds that form an exciplex preferablycontains a metal element with large spin-orbit interaction,specifically, a transition metal element. It is particularly preferablethat a platinum group element (ruthenium (Ru), rhodium (Rh), palladium(Pd), osmium (Os), iridium (Ir), or platinum (Pt)), especially iridium,be contained because intersystem crossing between a singlet excitedstate and a triplet excited state in the exciplex can be increased.

In the above structure, a material with a high luminescence quantumyield does not need to be used to form an exciplex; accordingly, thedesign of a material is facilitated and a material can be selected froma wide range of choices. Specifically, in the case where at least one ofthe compounds that form an exciplex has a function of converting tripletexcitation energy into light emission by itself or in the case where atleast one of the compounds contains a platinum group element (Ru, Rh,Pd, Os, Ir, or Pt), the luminescence quantum yield of the compound maybe higher than or equal to 0% and lower than or equal to 50%, higherthan or equal to 0% and lower than or equal to 40%, higher than or equalto 0% and lower than or equal to 25%, higher than or equal to 0% andlower than or equal to 10%, or higher than or equal to 0% and lower thanor equal to 1%, at room temperature or normal temperature.

It is difficult for a general host-guest light-emitting elementcontaining a phosphorescent compound to emit light with a desired colorat high efficiency. In contrast, since an exciplex is used as alight-emitting material in one embodiment of the present invention, adesired emission color can be obtained and the efficiency can beincreased.

FIG. 2A is a schematic cross-sectional view illustrating an example ofthe light-emitting layer 130 in FIG. 1. The light-emitting layer 130illustrated in FIG. 2A contains a compound 131 and a compound 132. Inone embodiment of the present invention, the compound 131 is preferablya phosphorescent compound. Although a structure in which aphosphorescent compound is used as the compound 131 is described below,a compound that does not emit light at room temperature may be used aslong as the compound contains a platinum group element.

The compound 131 and the compound 132 contained in the light-emittinglayer 130 preferably form an exciplex.

Although it is acceptable as long as the compounds 131 and 132 can forman exciplex, it is preferable that one of them be a compound having afunction of transporting holes (a hole-transport property) and the otherbe a compound having a function of transporting electrons (anelectron-transport property). In that case, a donor-acceptor exciplex iseasily formed; thus, efficient formation of an exciplex is possible. Inthe case where the compounds 131 and 132 are a compound having ahole-transport property and a compound having an electron-transportproperty, the carrier balance can be easily controlled by adjusting themixture ratio. Specifically, the weight ratio of the compound having ahole-transport property to the compound having an electron-transportproperty is preferably within a range of 1:9 to 9:1. Since the carrierbalance can be easily controlled with the structure, a carrierrecombination region can also be controlled easily.

In order for the compounds 131 and 132 to efficiently form an exciplex,the highest occupied molecular orbital (also referred to as HOMO) levelof one of the compounds 131 and 132 is preferably higher than the HOMOlevel of the other of the compounds 131 and 132, and the lowestunoccupied molecular orbital (also referred to as LUMO) level of one ofthe compounds 131 and 132 is preferably higher than the LUMO level ofthe other of the compounds 131 and 132. Note that the HOMO level of thecompound 131 may be equivalent to the HOMO level of the compound 132, orthe LUMO level of the compound 131 may be equivalent to the LUMO levelof the compound 132.

Note that the LUMO levels and the HOMO levels of the compounds can bederived from the electrochemical characteristics (reduction potentialsand oxidation potentials) of the compounds that are measured by cyclicvoltammetry (CV).

When the compound 131 has a hole-transport property and the compound 132has an electron-transport property, for example, it is preferable thatthe HOMO level of the compound 131 be higher than the HOMO level of thecompound 132 and the LUMO level of the compound 131 be higher than theLUMO level of the compound 132, as in an energy band diagram in FIG. 2B.Such correlation of energy levels is suitable because electrons andholes, which serve as carriers, from the pair of electrodes (theelectrodes 101 and 102) are easily injected into the compound 131 andthe compound 132, respectively.

In FIG. 2B, Comp (131) represents the compound 131, Comp (132)represents the compound 132, ΔE_(C1) represents the energy differencebetween the LUMO level and the HOMO level of the compound 131, ΔE_(C2)represents the energy difference between the LUMO level and the HOMOlevel of the compound 132, and ΔE_(Ex) represents the energy differencebetween the LUMO level of the compound 132 and the HOMO level of thecompound 131.

The exciplex formed by the compound 131 and the compound 132 has HOMO inthe compound 131 and LUMO in the compound 132. The excitation energy ofthe exciplex substantially corresponds to the energy difference betweenthe LUMO level of the compound 132 and the HOMO level of the compound131 (ΔF_(Ex)), which is smaller than the energy difference between theLUMO level and the HOMO level of the compound 131 (ΔE_(C1)) and theenergy difference between the LUMO level and the HOMO level of thecompound 132 (ΔE_(C2)). Thus, when the compound 131 and the compound 132form an exciplex, an excited state can be formed with lower excitationenergy. Having lower excitation energy, the exciplex can form a stableexcited state.

FIG. 2C shows the correlation of the energy levels of the compounds 131and 132 in the light-emitting layer 130. The following explains whatterms and signs in FIG. 2C represent:

Comp (131): the compound 131 (phosphorescent compound);

Comp (132): the compound 132;

S_(C1): the S1 level of the compound 131;

T_(C1): the T1 level of the compound 131;

S_(C2): the S1 level of the compound 132;

T_(C2): the T1 level of the compound 132;

S_(Ex): the S1 level of the exciplex; and

T_(Ex): the T1 level of the exciplex.

In the light-emitting element of one embodiment of the presentinvention, the compound 131 and the compound 132 contained in thelight-emitting layer 130 form the exciplex. The S1 level of the exciplex(S_(Ex)) and the T1 level of the exciplex (T_(Ex)) are close to eachother (see Route A₁ in FIG. 2C).

An exciplex is an excited state formed from two kinds of materials andis formed mainly through either of the following two processes.

One of the processes is a formation process of an electroplex. In thisspecification and the like, an electroplex refers to an exciplex formedby the interaction between the ionized compounds 131 and 132 (in acation state or an anion state) formed due to carrier injection. Notethat the formation process of an electroplex occurs in electricalexcitation. In this embodiment, the formation process of an electroplexcorresponds to a process in which one of the compounds 131 and 132accepts holes and the other accepts electrons and the compounds 131 and132 interact with each other so that an exciplex is formed immediately.In the formation process of an electroplex, an exciplex is formedimmediately and an excited state is not formed by the compound 131 orthe compound 132 by itself. Thus, the excitation lifetime and theluminescence quantum yields of the compounds (the compounds 131 and 132)that form an exciplex do not affect the formation process of anexciplex. In other words, one embodiment of the present invention canform an exciplex efficiently even when the luminescence quantum yieldsof the compounds (the compounds 131 and 132) that form an exciplex arelow. Note that in this formation process, the S1 level of the compound131 (S_(C1)) may be higher or lower than the S1 level of the compound132 (S_(C2)), and the T1 level of the compound 131 (T_(C1)) may behigher or lower than the T1 level of the compound 132 (T_(C2)).

The other of the processes is a process for forming an exciplex when onecompound that is brought into an excited state by receiving excitationenergy interacts with the other compound in a ground state. Thisformation process of an exciplex may occur in photoexcitation andelectrical excitation. In this embodiment, the formation process of anexciplex corresponds to a process in which one of the compounds 131 and132 receives light or electric energy to be brought into an excitedstate and interacts with the other in a ground state immediately to forman exciplex. In this formation process of an exciplex, excitation energycan be transferred from the T1 level of the compound 131 (T_(C1)) to theT1 level of the compound 132 (T_(C2)) even when the compound 131 isbrought into an excited state and the deactivation rate of the excitedstate is fast, as long as the T1 level of the compound 131 (T_(C1)) ishigher than or equal to the T1 level of the compound 132 (T_(C2)). Afterthe excitation energy is transferred from the T1 level of the compound131 (T_(C1)) to the T1 level of the compound 132 (T_(C2)), the compound131 and the compound 132 can form an exciplex. Thus, one embodiment ofthe present invention can form an exciplex efficiently even when thedeactivation rate of the excited state of the compound 131 is fast andthe luminescence quantum yield of the compound 131 is low. Note that inthis formation process, the S1 level of the compound 131 (S_(C1)) may behigher or lower than the S1 level of the compound 132 (S_(C2)).

When an exciplex formed through either of the above processes losesexcitation energy by light emission or the like to be brought into aground state, two kinds materials forming the exciplex serve as theoriginal two kinds of materials.

Because the excitation energy levels of the exciplex (S_(Ex) and T_(Ex))are lower than the S1 levels of the materials (the compounds 131 and132) that form an exciplex (S_(C1) and S_(C2)), an excited state can beformed with lower excitation energy. Accordingly, the driving voltage ofthe light-emitting element 150 can be reduced.

Since the S1 level and the T1 level of an exciplex (S_(Ex) and T_(Ex))are adjacent to each other, the exciplex has a function of emittingthermally activated delayed fluorescence. In other words, the exciplexhas a function of converting triplet excitation energy into singletexcitation energy by reverse intersystem crossing (upconversion). Thus,the triplet excitation energy generated in the light-emitting layer 130is partly converted into singlet excitation energy by the exciplex. Inorder to cause this conversion, the energy difference between the S1level and the T1 level of the exciplex (S_(Ex) and T_(Ex)) is preferablygreater than 0 eV and less than or equal to 0.2 eV, and furtherpreferably greater than 0 eV and less than or equal to 0.1 eV. Note thatin order to efficiently cause reverse intersystem crossing, the T1 levelof the exciplex (T_(Ex)) is preferably lower than the T1 levels of thematerials (the compounds 131 and 132) that form an exciplex (T_(C1) andT_(C2)). In that case, quenching of the triplet excitation energy of theexciplex formed by the compound 131 and the compound 132 is less likelyto occur, leading to efficient reverse intersystem crossing from tripletexcitation energy to singlet excitation energy by the exciplex and thesubsequent light emission from the singlet excitation energy.

A compound containing a heavy atom is used as one of the compounds thatform an exciplex in one embodiment of the present invention, whichpromotes intersystem crossing between a singlet state and a tripletstate. Thus, an exciplex that emits light (phosphorescence) from tripletexcitation energy can be formed. Note that it is difficult to clearlydistinguish fluorescence and phosphorescence from each other in anemission spectrum in some cases because the S1 level and the T1 level ofthe exciplex (S_(Ex) and T_(Ex)) are adjacent to each other. In thatcase, fluorescence and phosphorescence can be sometimes distinguishedfrom each other by the emission lifetime.

To make the formation process of an electroplex more likely to occur,when the compound 131 has a hole-transport property and the compound 132has an electron-transport property, for example, it is preferable thatthe HOMO level of the compound 131 be higher than the HOMO level of thecompound 132 and the LUMO level of the compound 131 be higher than theLUMO level of the compound 132. Specifically, the energy differencebetween the HOMO levels of the compound 131 and the compound 132 ispreferably greater than or equal to 0.1 eV, further preferably greaterthan or equal to 0.2 eV, and still further preferably greater than orequal to 0.3 eV. Moreover, the energy difference between the LUMO levelsof the compound 131 and the compound 132 is preferably greater than orequal to 0.1 eV, further preferably greater than or equal to 0.2 eV, andstill further preferably greater than or equal to 0.3 eV. Such energydifferences are suitable because electrons and holes, which serve ascarriers, from the pair of electrodes (the electrodes 101 and 102) areeasily injected into the compound 131 and the compound 132,respectively.

The compound 131 may have an electron-transport property and thecompound 132 may have a hole-transport property. In that case, the HOMOlevel of the compound 132 is preferably higher than the HOMO level ofthe compound 131 and the LUMO level of the compound 132 is preferablyhigher than the LUMO level of the compound 131, as in an energy banddiagram in FIG. 4A.

The weight ratio of the compound 131 to the compound 132 is preferablylow. Specifically, the weight ratio of the compound 131 to the compound132 is preferably greater than or equal to 0.01 and less than or equalto 0.5, and further preferably greater than or equal to 0.05 and lessthan or equal to 0.3. A weight ratio within the above range ispreferable because the formation process of an electroplex can be morelikely to occur than a direct recombination process of carriers in oneof the compounds.

<Light Emission Mechanism 2 of Light-Emitting Element>

Next, an example of a structure that is different from that of thelight-emitting layer illustrated in FIG. 2A will be described below withreference to FIG. 3A.

FIG. 3A is a schematic cross-sectional view illustrating an example ofthe light-emitting layer 130 in FIG. 1. The light-emitting layer 130illustrated in FIG. 3A contains the compound 131, the compound 132, anda compound 133.

In the light-emitting layer 130, the compound 132 or the compound 133 ispresent in the highest proportion by weight, and the compound 131 isdispersed in the compound 132 and the compound 133. Although thecompound 131 is preferably a phosphorescent compound, a compound thatdoes not emit light at room temperature may be used as long as thecompound contains a platinum group element. The compound 131 and thecompound 132 preferably form an exciplex.

To efficiently form an exciplex, one of the compounds 131 and 132preferably has the highest HOMO level among the materials of thelight-emitting layer 130, and the other of the compounds 131 and 132preferably has the lowest LUMO level among the materials of thelight-emitting layer 130. In other words, the HOMO level of one of thecompounds 131 and 132 is preferably higher than the HOMO level of theother of the compounds 131 and 132 and the HOMO level of the compound133, and the LUMO level of the other of the compounds 131 and 132 ispreferably lower than the LUMO level of one of the compounds 131 and 132and the LUMO level of the compound 133. With such correlation of theenergy levels, a reaction for forming an exciplex by the compound 132and the compound 133 can be inhibited.

When the compound 131 has a hole-transport property and the compound 132has an electron-transport property, for example, it is preferable thatthe HOMO level of the compound 131 be higher than the HOMO levels of thecompounds 132 and 133, and the LUMO level of the compound 132 be lowerthan the LUMO levels of the compounds 131 and 133, as in an energy banddiagram in FIG. 3B. The LUMO level of the compound 133 may be higher orlower than the LUMO level of the compound 131. The HOMO level of thecompound 133 may be higher or lower than the HOMO level of the compound132.

In FIG. 3B, Comp (131) represents the compound 131, Comp (132)represents the compound 132, Comp (133) represents the compound 133,ΔE_(C1) represents the energy difference between the LUMO level and theHOMO level of the compound 131, ΔE_(C2) represents the energy differencebetween the LUMO level and the HOMO level of the compound 132, ΔE_(C3)represents the energy difference between the LUMO level and the HOMOlevel of the compound 133, and ΔF_(Ex) represents the energy differencebetween the LUMO level of the compound 132 and the HOMO level of thecompound 131.

The excitation energy of the exciplex formed by the compound 131 and thecompound 132 substantially corresponds to the energy difference betweenthe LUMO level of the compound 132 and the HOMO level of the compound131 (ΔE_(Ex)) and is preferably smaller than the energy differencebetween the LUMO level and the HOMO level of the compound 133 (ΔE_(C3)).

FIG. 3C shows the correlation of the energy levels of the compounds 131,132, and 133 in the light-emitting layer 130 in FIG. 3A. The followingexplains what terms and signs in FIG. 3C represent:

Comp (131): the compound 131 (phosphorescent compound);

Comp (132): the compound 132;

Comp (133): the compound 133;

S_(C1): the S1 level of the compound 131;

T_(C1): the T1 level of the compound 131;

S_(C2): the S1 level of the compound 132;

T_(C2): the T1 level of the compound 132;

S_(C3): the S1 level of the compound 133;

T_(C3): the T1 level of the compound 133;

S_(Ex): the S1 level of the exciplex; and

T_(Ex): the T1 level of the exciplex.

In the light-emitting element of one embodiment of the presentinvention, the compound 131 and the compound 132 contained in thelight-emitting layer 130 form the exciplex. The S1 level of the exciplex(S_(Ex)) and the T1 level of the exciplex (T_(Ex)) are close to eachother (see Route A₁ in FIG. 3C).

The T1 level of the compound 133 (T_(C3)) is preferably higher than orequal to the T1 level of the compound 131 (T_(C1)). The T1 level of thecompound 131 (T_(C1)) is preferably higher than or equal to the T1 levelof the exciplex (T_(Ex)) formed by the compound 131 and the compound132. With such correlation of the energy levels, quenching of theexcitation energy of the exciplex formed by the compound 131 and thecompound 132 is less likely to occur, leading to efficient lightemission from triplet excitation energy or efficient reverse intersystemcrossing from the triplet excitation energy to singlet excitation energyby the exciplex and the subsequent light emission from the singletexcitation energy.

To make the formation process of an electroplex more likely to occur, itis preferable to add a material other than the compounds 131 and 132into the light-emitting layer 130 so that the direct recombinationprocess of carriers is less likely to occur in the compound 131. Thismeans that when the light-emitting layer 130 contains the compound 133in addition to the compounds 131 and 132, an electroplex is likely to beformed by the compound 131 and the compound 132. The weight ratio of thecompound 131 to the total amount of the compounds 132 and 133 ispreferably low. Specifically, the weight ratio of the compound 131 tothe total amount of the compounds 132 and 133 is preferably greater thanor equal to 0.01 and less than or equal to 0.5, and further preferablygreater than or equal to 0.05 and less than or equal to 0.3.

The compound 131 may have an electron-transport property and thecompound 132 may have a hole-transport property. In that case, the HOMOlevel of the compound 132 is preferably higher than the HOMO levels ofthe compounds 131 and 133 and the LUMO level of the compound 131 ispreferably lower than the LUMO levels of the compounds 132 and 133, asin an energy band diagram in FIG. 4B. The LUMO level of the compound 133may be higher or lower than the LUMO level of the compound 132. The HOMOlevel of the compound 133 may be higher or lower than the HOMO level ofthe compound 131.

<Material>

Next, components of a light-emitting element of one embodiment of thepresent invention will be described in detail below.

<<Light-Emitting Layer>>

Materials that can be used for the light-emitting layer 130 will bedescribed below.

Although there is no particular limitation on the compound 131 and thecompound 132 as long as an exciplex can be formed, it is preferable thatone of them have a function of transporting electrons and the other havea function of transporting holes.

In the case where the compound 132 has a function of transporting holes,the compound 132 preferably includes at least one of a π-electron richheteroaromatic skeleton and an aromatic amine skeleton.

As the π-electron rich heteroaromatic skeleton included in the compound132, one or more of a furan skeleton, a thiophene skeleton, and apyrrole skeleton are preferable because of their high stability andreliability. As a furan skeleton, a dibenzofuran skeleton is preferable.As a thiophene skeleton, a dibenzothiophene skeleton is preferable. Notethat as a pyrrole skeleton, an indole skeleton, a carbazole skeleton, ora 3-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole skeleton is preferable.Each of these skeletons may further have a substituent.

As the aromatic amine skeleton included in the compound 132, tertiaryamine not including an NH bond, in particular, a triarylamine skeletonis preferably used. As an aryl group of a triarylamine skeleton, asubstituted or unsubstituted aryl group having 6 to 13 carbon atomsincluded in a ring is preferably used and examples thereof include aphenyl group, a naphthyl group, and a fluorenyl group.

A structure including a π-electron rich heteroaromatic skeleton and anaromatic amine skeleton, which has an excellent hole-transport propertyand thus is stable and highly reliable, is particularly preferred. Anexample of such a structure is a structure including a carbazoleskeleton and an arylamine skeleton.

As examples of the above-described π-electron rich heteroaromaticskeleton and aromatic amine skeleton, skeletons represented by thefollowing general formulae (101) to (117) are given. Note that X in thegeneral formulae (115) to (117) represents an oxygen atom or a sulfuratom.

In the case where the compound 132 has a function of transportingelectrons, the compound 132 preferably includes a π-electron deficientheteroaromatic skeleton. As the π-electron deficient heteroaromaticskeleton, a pyridine skeleton, a diazine skeleton (a pyrimidineskeleton, a pyrazine skeleton, or a pyridazine skeleton), or a triazineskeleton is preferable; the diazine skeleton or the triazine skeleton isparticularly preferable because of its high stability and reliability.

As examples of the above-described π-electron deficient heteroaromaticskeleton, skeletons represented by the following general formulae (201)to (218) are given. Note that X in General Formulae (209) to (211)represents an oxygen atom or a sulfur atom.

Alternatively, a compound may be used in which a skeleton having ahole-transport property (e.g., at least one of a π-electron richheteroaromatic skeleton and an aromatic amine skeleton) and a skeletonhaving an electron-transport property (e.g., a π-electron deficientheteroaromatic skeleton) are bonded to each other directly or through anarylene group. Examples of the above-described arylene group include aphenylene group, a biphenyldiyl group, a naphthalenediyl group, and afluorenediyl group.

As examples of a bonding group which bonds the above skeleton having ahole-transport property and the above skeleton having anelectron-transport property, groups represented by the following generalformulae (301) to (315) are given.

The above aromatic amine skeleton (e.g., the triarylamine skeleton), theabove π-electron rich heteroaromatic skeleton (e.g., a ring includingthe furan skeleton, the thiophene skeleton, or the pyrrole skeleton),and the above π-electron deficient heteroaromatic skeleton (e.g., a ringincluding the diazine skeleton or the triazine skeleton) or the abovegeneral formulae (101) to (117), (201) to (218), and (301) to (315) mayeach have a substituent. As the substituent, an alkyl group having 1 to6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or asubstituted or unsubstituted aryl group having 6 to 12 carbon atoms canbe selected. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 6 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 12 carbon atoms are a phenylgroup, a naphthyl group, a biphenyl group, and the like. The abovesubstituents may be bonded to each other to form a ring. For example, inthe case where a carbon atom at the 9-position in a fluorene group hastwo phenyl groups as substituents, the phenyl groups are bonded to forma spirofluorene skeleton. Note that an unsubstituted group has anadvantage in easy synthesis and an inexpensive raw material.

Furthermore, Ar represents a single-bond arylene group or an arylenegroup having 6 to 13 carbon atoms. The arylene group may include one ormore substituents and the substituents may be bonded to each other toform a ring. For example, a carbon atom at the 9-position in a fluorenylgroup has two phenyl groups as substituents and the phenyl groups arebonded to form a spirofluorene skeleton. Specific examples of thearylene group having 6 to 13 carbon atoms are a phenylene group, anaphthalenediyl group, a biphenyldiyl group, a fluorenediyl group, andthe like. In the case where the arylene group has a substituent, as thesubstituent, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 6 carbon atoms, or an aryl group having 6 to 12 carbonatoms can be selected. Specific examples of the alkyl group having 1 to6 carbon atoms include a methyl group, an ethyl group, a propyl group,an isopropyl group, a butyl group, an isobutyl group, a tert-butylgroup, an n-hexyl group, and the like. Specific examples of a cycloalkylgroup having 3 to 6 carbon atoms include a cyclopropyl group, acyclobutyl group, a cyclopentyl group, a cyclohexyl group, and the like.Specific examples of the aryl group having 6 to 12 carbon atoms are aphenyl group, a naphthyl group, a biphenyl group, and the like.

As the arylene group represented by Ar, for example, groups representedby structural formulae (Ar-1) to (Ar-18) below can be used. Note thatthe group that can be used as Ar is not limited to these.

Furthermore, R¹ and R² each independently represent any of hydrogen, analkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms. Specific examples of the alkyl group having 1 to 6carbon atoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 6 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms are a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike. The above aryl group or phenyl group may include one or moresubstituents, and the substituents may be bonded to each other to form aring. As the substituent, an alkyl group having 1 to 6 carbon atoms, acycloalkyl group having 3 to 6 carbon atoms, or an aryl group having 6to 12 carbon atoms can be selected. Specific examples of the alkyl grouphaving 1 to 6 carbon atoms include a methyl group, an ethyl group, apropyl group, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, an n-hexyl group, and the like. Specific examples of acycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, and thelike. Specific examples of the aryl group having 6 to 12 carbon atomsinclude a phenyl group, a naphthyl group, a biphenyl group, and thelike.

For example, groups represented by structural formulae (R-1) to (R-29)below can be used as the alkyl group or aryl group represented by R¹ andR². Note that the groups which can be used as an alkyl group or an arylgroup are not limited thereto.

As a substituent that can be included in the general formulae (101) to(117), (201) to (218), and (301) to (315), Ar, R¹, and R², the alkylgroup or aryl group represented by the above structural formulae (R-1)to (R-24) can be used, for example. Note that the group which can beused as an alkyl group or an aryl group is not limited thereto.

As the compound 132, any of the following hole-transport materials andelectron-transport materials can be used, for example.

A material having a property of transporting more holes than electronscan be used as the hole-transport material, and a material having a holemobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Specifically, anaromatic amine, a carbazole derivative, or the like can be used.Furthermore, the hole-transport material may be a high molecularcompound.

Examples of the aromatic amine compound, which has a high hole-transportproperty, include N,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine(abbreviation: DTDPPA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

Specific examples of the carbazole derivative are3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA1),3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA2),3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole(abbreviation: PCzTPN2),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

Other examples of the carbazole derivative are4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),1,4-bis[4-(N-carbazoly)phenyl]-2,3,5,6-tetraphenylbenzene, and the like.

Examples of the material having an excellent hole-transport property arearomatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA),4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1-TNATA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation:TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: m-MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N′-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine(abbreviation: DFLADFL),N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine(abbreviation: DPNF),2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPASF),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBiA1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),4-phenyldiphenyl-(9-phenyl-9H-carbazol-3-yl)amine (abbreviation:PCA1BP), N,N-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine(abbreviation: PCA2B),N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine(abbreviation: PCA3B),N-(4-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carbazol-3-amine(abbreviation: PCBiF),N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: PCBBiF),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF),N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF),2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF),2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPA2SF),N-[4-(9H-carbazol-9-yl)phenyl]-N-(4-phenyl)phenylaniline (abbreviation:YGA1BP), andN,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine(abbreviation: YGA2F). Other examples are amine compounds, carbazolecompounds, thiophene compounds, furan compounds, fluorene compounds;triphenylene compounds; phenanthrene compounds, and the like such as3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPN),3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation:PCPPn), 3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP),1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP),4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II),4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II),1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviated as DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III),4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV), and4-[3-(triphenylen-2-yl)phenyl]dibenzothiophene (abbreviation:mDBTPTp-II). The substances described here are mainly substances havinga hole mobility of 1×10⁻⁶ cm²/Vs or higher. Note that other than thesesubstances, any substance that has a property of transporting more holesthan electrons may be used.

As the electron-transport material, a material having a property oftransporting more electrons than holes can be used, and a materialhaving an electron mobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Ametal complex containing zinc or aluminum, a π-electron deficientheteroaromatic compound such as a nitrogen-containing heteroaromaticcompound, or the like can be used as a material which easily acceptselectrons (material having an electron-transport property). As the metalcomplex, a metal complex having a quinoline ligand, a benzoquinolineligand, an oxazole ligand, or a thiazole ligand can be used. As theπ-electron deficient heteroaromatic compound, an oxadiazole derivative,a triazole derivative, a phenanthroline derivative, a pyridinederivative, a bipyridine derivative, a pyrimidine derivative, a triazinederivative, or the like can be used.

Examples include metal complexes having a quinoline or benzoquinolineskeleton, such as tris(8-quinolinolato)aluminum(III) (abbreviation:Alq), tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),and the like. Alternatively, a metal complex having an oxazole-based orthiazole-based ligand, such as bis[2-(2-benzoxazolyl)phenolato]zinc(II)(abbreviation: ZnPBO) or bis[2-(2-benzothiazoly)phenolato]zinc(II)(abbreviation: ZnBTZ), can be used. Other than such metal complexes, anyof the following can be used: heterocyclic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI),2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), and2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation:NBPhen); heterocyclic compounds having a diazine skeleton, such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq),2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2CzPDBq-III),7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:7mDBTPDBq-II), 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 6mDBTPDBq-II),4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation:4,6mPnP2Pm), 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine(abbreviation: 4,6mDBTP2Pm-II), and4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation:4,6mCzP2Pm); heterocyclic compounds having a triazine skeleton, such as2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine(abbreviation: PCCzPTzn); heterocyclic compounds having a pyridineskeleton, such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine(abbreviation: 35DCzPPy) and 1,3,5-tri[3-(3-pyridyl)phenyl]benzene(abbreviation: TmPyPB); and heteroaromatic compounds such as4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Stillalternatively, a high molecular compound such as poly(2,5-pyridinediyl)(abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used. The substances described here aremainly substances having an electron mobility of 1×10⁻⁶ cm²/Vs orhigher. Note that other substances may also be used as long as theirelectron-transport properties are more excellent than theirhole-transport properties.

As the compound 131 (phosphorescent compound), an iridium-, rhodium-, orplatinum-based organometallic or metal complex, or a platinum ororganoiridium complex having a porphyrin ligand can be used; it isparticularly preferable to use an organoiridium complex such as aniridium-based ortho-metalated complex. As an ortho-metalated ligand, a4H-triazole ligand, a 1H-triazole ligand, an imidazole ligand, apyridine ligand, a pyrimidine ligand, a pyrazine ligand, an isoquinolineligand, or the like can be used. Here, the compound 131 (phosphorescentcompound) has an absorption band of triplet metal to ligand chargetransfer (MLCT) transition.

Examples of the substance that has an emission peak in the blue or greenwavelength range include organometallic iridium complexes having a4H-triazole skeleton, such astris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III) (abbreviation: Ir(mpptz-dmp)₃),tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Mptz)₃),tris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPrptz-3b)₃), andtris[3-(5-biphenyl)-5-isopropyl-4-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPr5btz)₃); organometallic iridium complexes having a1H-triazole skeleton, such astris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(Mptzl-mp)₃) andtris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Prptzl-Me)₃); organometallic iridium complexes havingan imidazole skeleton, such asfac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III)(abbreviation: Ir(iPrpmi)₃) andtris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III)(abbreviation: Ir(dmpimpt-Me)₃); and organometallic iridium complexes inwhich a phenylpyridine derivative having an electron-withdrawing groupis a ligand, such asbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: FIrpic),bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)), andbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIr(acac)). Among the materials givenabove, the organometallic iridium complexes including anitrogen-containing five-membered heterocyclic skeleton, such as a4H-triazole skeleton, a 1H-triazole skeleton, or an imidazole skeletonhave high triplet excitation energy, reliability, and luminousefficiency and are thus especially preferable.

Examples of the substance that has an emission peak in the green oryellow wavelength range include organometallic iridium complexes havinga pyrimidine skeleton, such astris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation:Ir(mppm)₃), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₃),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(mppm)₂(acac)),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)),(acetylacetonato)bis[4-(2-norbornyl)-6-phenylpyrimidinato]iridium(III)(endo-and exo-mixture) (abbreviation: Ir(nbppm)₂(acac)),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: Ir(mpmppm)₂(acac)),(acetylacetonato)bis{4,6-dimethyl-2-[6-(2,6-dimethylphenyl)-4-pyrimidinyl-κN³]phenyl-κC}iridium(III)(abbreviation: Ir(dmppm-dmp)₂(acac)), and(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: Ir(dppm)₂(acac)); organometallic iridium complexes havinga pyrazine skeleton, such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)) and(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-iPr)₂(acac)); organometallic iridium complexeshaving a pyridine skeleton, such astris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(ppy)₂(acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)₂(acac)),tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)₃),tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(pq)₃),and bis(2-phenylquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(pq)₂(acac)); organometallic iridium complexes such asbis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C^(2′)}iridium(III)acetylacetonate (abbreviation: Ir(p-PF-ph)₂(acac)), andbis(2-phenylbenzothiazolato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(bt)₂(acac)); and a rare earth metal complex such astris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)). Among the materials given above, the organometalliciridium complexes having a pyrimidine skeleton have distinctively highreliability and luminous efficiency and are thus particularlypreferable.

Examples of the substance that has an emission peak in the yellow or redwavelength range include organometallic iridium complexes having apyrimidine skeleton, such as(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III)(abbreviation: Ir(5mdppm)₂(dibm)),bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(5mdppm)₂(dpm)), and(dipivaloylmethanato)bis[4,6-di(naphthalen-1-yl)pyrimidinato]iridium(III)(abbreviation: Ir(dlnpm)₂(dpm)); organometallic iridium complexes havinga pyrazine skeleton, such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)), bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: Ir(tppr)₂(dpm)), and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)); organometallic iridium complexes havinga pyridine skeleton, such astris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation:Ir(piq)₃) and bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(piq)₂(acac)); a platinum complex suchas 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP); and rare earth metal complexes such astris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)) andtris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)). Among the materials given above, theorganometallic iridium complexes having a pyrimidine skeleton havedistinctively high reliability and luminous efficiency and are thusparticularly preferable. Furthermore, the organometallic iridiumcomplexes having a pyrazine skeleton can provide red light emission withfavorable chromaticity.

Although there is no particular limitation on a material that can beused as the compound 133 in the light-emitting layer 130, any of thefollowing materials can be used, for example: metal complexes such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h] quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), andbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ);heterocyclic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), and9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11); and aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB). In addition, condensed polycyclic aromaticcompounds such as anthracene derivatives, phenanthrene derivatives,pyrene derivatives, chrysene derivatives, and dibenzo[g,p]chrysenederivatives can be given, and specific examples are9,10-diphenylanthracene (abbreviation: DPAnth),N,N-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N,9-diphenyl-N-(9,10-diphenyl-2-anthryl)-9H-carbazol-3-amine(abbreviation: 2PCAPA), 6,12-dimethoxy-5,11-diphenylchrysene,N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2),1,3,5-tri(1-pyrenyl)benzene (abbreviation: TPB3), and the like. One ormore substances having a wider energy gap than the compound 131 areselected from these substances and known substances.

The light-emitting layer 130 can include two or more layers. Forexample, in the case where the light-emitting layer 130 is formed bystacking a first light-emitting layer and a second light-emitting layerin this order from the hole-transport layer side, the firstlight-emitting layer is formed using a substance having a hole-transportproperty as the host material and the second light-emitting layer isformed using a substance having an electron-transport property as thehost material.

The light-emitting layer 130 may contain a material other than thecompounds 131, 132, and 133.

<<Pair of Electrodes>>

The electrode 101 and the electrode 102 have functions of injectingholes and electrons into the light-emitting layer 130. The electrode 101and the electrode 102 can be formed using a metal, an alloy, or aconductive compound, a mixture or a stack thereof, or the like. Atypical example of the metal is aluminum (Al); besides, a transitionmetal such as silver (Ag), tungsten, chromium, molybdenum, copper, ortitanium, an alkali metal such as lithium (Li) or cesium, or a Group 2metal such as calcium or magnesium (Mg) can be used. As a transitionmetal, a rare earth metal such as ytterbium (Yb) may be used. An alloycontaining any of the above metals can be used as the alloy, and MgAgand AlLi can be given as examples. Examples of the conductive compoundinclude metal oxides such as indium tin oxide (hereinafter referred toas ITO), indium tin oxide containing silicon or silicon oxide (ITSO),indium zinc oxide, indium oxide containing tungsten and zinc, and thelike. It is also possible to use an inorganic carbon-based material suchas graphene as the conductive compound. As described above, theelectrode 101 and/or the electrode 102 may be formed by stacking two ormore of these materials.

Light emitted from the light-emitting layer 130 is extracted through theelectrode 101 and/or the electrode 102. Therefore, at least one of theelectrodes 101 and 102 transmits visible light. As the conductivematerial transmitting light, a conductive material having a visiblelight transmittance higher than or equal to 40% and lower than or equalto 100%, preferably higher than or equal to 60% and lower than or equalto 100%, and a resistivity lower than or equal to 1×10⁻² Ω·cm can beused. The electrode on the light extraction side may be formed using aconductive material having functions of transmitting light andreflecting light. As the conductive material, a conductive materialhaving a visible light reflectivity higher than or equal to 20% andlower than or equal to 80%, preferably higher than or equal to 40% andlower than or equal to 70%, and a resistivity lower than or equal to1×10⁻² Ω·cm can be used. In the case where the electrode through whichlight is extracted is formed using a material with low lighttransmittance, such as metal or alloy, the electrode 101 and/or theelectrode 102 is formed to a thickness that is thin enough to transmitvisible light (e.g., a thickness of 1 nm to 10 nm).

In this specification and the like, as the electrode transmitting light,a material that transmits visible light and has conductivity is used.Examples of the material include, in addition to the above-describedoxide conductor layer typified by an ITO, an oxide semiconductor layerand an organic conductor layer containing an organic substance. Examplesof the organic conductive layer containing an organic substance includea layer containing a composite material in which an organic compound andan electron donor (donor material) are mixed and a layer containing acomposite material in which an organic compound and an electron acceptor(acceptor material) are mixed. The resistivity of the transparentconductive layer is preferably lower than or equal to 1×10⁵ Ω·cm,further preferably lower than or equal to 1×10⁴ Ω·cm.

As the method for forming the electrode 101 and the electrode 102, asputtering method, an evaporation method, a printing method, a coatingmethod, a molecular beam epitaxy (MBE) method, a CVD method, a pulsedlaser deposition method, an atomic layer deposition (ALD) method, or thelike can be used as appropriate.

<<Hole-Injection Layer>>

The hole-injection layer 111 has a function of reducing a barrier forhole injection from one of the pair of electrodes (the electrode 101 orthe electrode 102) to promote hole injection and is formed using atransition metal oxide, a phthalocyanine derivative, or an aromaticamine, for example. As the transition metal oxide, molybdenum oxide,vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or thelike can be given. As the phthalocyanine derivative, phthalocyanine,metal phthalocyanine, or the like can be given. As the aromatic amine, abenzidine derivative, a phenylenediamine derivative, or the like can begiven. It is also possible to use a high molecular compound such aspolythiophene or polyaniline; a typical example thereof ispoly(ethylenedioxythiophene)/poly(styrenesulfonic acid), which isself-doped polythiophene.

As the hole-injection layer 111, a layer containing a composite materialof a hole-transport material and a material having a property ofaccepting electrons from the hole-transport material can also be used.Alternatively, a stack of a layer containing a material having anelectron accepting property and a layer containing a hole-transportmaterial may also be used. In a steady state or in the presence of anelectric field, electric charge can be transferred between thesematerials. As examples of the material having an electron-acceptingproperty, organic acceptors such as a quinodimethane derivative, achloranil derivative, and a hexaazatriphenylene derivative can be given.A specific example is a compound having an electron-withdrawing group (ahalogen group or a cyano group), such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F4TCNQ), chloranil, or2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT-CN). Alternatively, a transition metal oxide such as an oxide of ametal from Group 4 to Group 8 can also be used. Specifically, vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, rhenium oxide, or the like can be used.In particular, molybdenum oxide is preferable because it is stable inthe air, has a low hygroscopic property, and is easily handled.

A material having a property of transporting more holes than electronscan be used as the hole-transport material, and a material having a holemobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Specifically, any ofthe above aromatic amines, the above carbazole derivatives, the abovearomatic hydrocarbons, the above stilbene derivatives, and the like asexamples of the hole-transport material that can be used in thelight-emitting layer 130 can be used. Furthermore, the hole-transportmaterial may be a high molecular compound.

Examples of the aromatic hydrocarbon are2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, andthe like. Other examples are pentacene, coronene, and the like. Thearomatic hydrocarbon having a hole mobility of 1×10⁻⁶ cm²/Vs or higherand having 14 to 42 carbon atoms is particularly preferable.

The aromatic hydrocarbon may have a vinyl skeleton. Examples of thearomatic hydrocarbon having a vinyl group are4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi),9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA),and the like.

Other examples are high molecular compounds such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:poly-TPD).

<<Hole-Transport Layer>>

The hole-transport layer 112 is a layer containing a hole-transportmaterial and can be formed using any of the materials given as examplesof the material of the hole-injection layer 111. In order that thehole-transport layer 112 has a function of transporting holes injectedinto the hole-injection layer 111 to the light-emitting layer 130, theHOMO level of the hole-transport layer 112 is preferably equal or closeto the HOMO level of the hole-injection layer 111.

As the hole-transport material, any of the materials given as examplesof the material of the hole-injection layer 111 can be used. As thehole-transport material, a substance having a hole mobility of 1×10⁻⁶cm²/Vs or higher is preferably used. Note that any substance other thanthe above substances may be used as long as the hole-transport propertyis more excellent than the electron-transport property. The layerincluding a substance having an excellent hole-transport property is notlimited to a single layer, and two or more layers containing theaforementioned substances may be stacked.

<<Electron-Transport Layer>>

The electron-transport layer 118 has a function of transporting, to thelight-emitting layer 130, electrons injected from the other of the pairof electrodes (the electrode 101 or the electrode 102) through theelectron-injection layer 119. A material having a property oftransporting more electrons than holes can be used as theelectron-transport material, and a material having an electron mobilityof 1×10⁻⁶ cm²/Vs or higher is preferable. As the compound which easilyaccepts electrons (the material having an electron-transport property),a π-electron deficient heteroaromatic compound such as anitrogen-containing heteroaromatic compound, a metal complex, or thelike can be used, for example. Specifically, a metal complex having aquinoline ligand, a benzoquinoline ligand, an oxazole ligand, or athiazole ligand, which are described as the electron-transport materialsthat can be used in the light-emitting layer 130, can be given.Furthermore, an oxadiazole derivative; a triazole derivative, aphenanthroline derivative, a pyridine derivative, a bipyridinederivative, a pyrimidine derivative, and the like can be given. Asubstance having an electron mobility of 1×10⁻⁶ cm²/Vs or higher ispreferable. Note that other than these substances, any substance thathas a property of transporting more electrons than holes may be used forthe electron-transport layer. The electron-transport layer 118 is notlimited to a single layer, and may include stacked two or more layerscontaining the aforementioned substances.

Between the electron-transport layer 118 and the light-emitting layer130, a layer that controls transfer of electron carriers may beprovided. The layer that controls transfer of electron carriers isformed by addition of a small amount of a substance having an excellentelectron-trapping property to a material having an excellentelectron-transport property described above, and is capable of adjustingcarrier balance by suppressing transfer of electron carriers. Such astructure is very effective in preventing a problem (such as a reductionin element lifetime) caused when electrons pass through thelight-emitting layer.

<<Electron-Injection Layer>>

The electron-injection layer 119 has a function of reducing a barrierfor electron injection from the electrode 102 to promote electroninjection and can be formed using a Group 1 metal or a Group 2 metal, oran oxide, a halide, or a carbonate of any of the metals, for example.Alternatively, a composite material containing an electron-transportmaterial (described above) and a material having a property of donatingelectrons to the electron-transport material can also be used. As thematerial having an electron-donating property, a Group 1 metal, a Group2 metal, an oxide of any of the metals, or the like can be given.Specifically, an alkali metal, an alkaline earth metal, or a compoundthereof, such as lithium fluoride (LiF), sodium fluoride (NaF), cesiumfluoride (CsF), calcium fluoride (CaF₂), or lithium oxide (LiOx), can beused. Alternatively, a rare earth metal compound like erbium fluoride(ErF₃) can be used. Electride may be used for the electron-injectionlayer 119. Examples of the electride include a substance in whichelectrons are added at high concentration to calcium oxide-aluminumoxide. The electron-injection layer 119 can be formed using thesubstance that can be used for the electron-transport layer 118.

A composite material in which an organic compound and an electron donor(donor) are mixed may be used for the electron-injection layer 119. Sucha composite material is excellent in an electron-injection property andan electron-transport property because electrons are generated in theorganic compound by the electron donor. In this case, the organiccompound is preferably a material that is excellent in transporting thegenerated electrons. Specifically, the above-listed substances forforming the electron-transport layer 118 (e.g., the metal complexes andheteroaromatic compounds) can be used, for example. As the electrondonor, a substance showing an electron-donating property with respect tothe organic compound may be used. Specifically, an alkali metal, analkaline earth metal, and a rare earth metal are preferable, andlithium, cesium, magnesium, calcium, erbium, ytterbium, and the like aregiven. In addition, an alkali metal oxide or an alkaline earth metaloxide is preferable, and lithium oxide, calcium oxide, barium oxide, andthe like are given. A Lewis base such as magnesium oxide may be used. Anorganic compound such as tetrathiafulvalene (abbreviation: TTF) may beused.

Note that the light-emitting layer, the hole-injection layer, thehole-transport layer, the electron-transport layer, and theelectron-injection layer described above can each be formed by anevaporation method (including a vacuum evaporation method), an inkjetmethod, a coating method, a nozzle printing method, a gravure printingmethod, or the like. Other than the above-mentioned materials, aninorganic compound such as a quantum dot or a high molecular compound(e.g., an oligomer, a dendrimer, and a polymer) may be used in thelight-emitting layer, the hole-injection layer, the hole-transportlayer, the electron-transport layer, and the electron-injection layer.

The quantum dot may be a colloidal quantum dot, an alloyed quantum dot,a core-shell quantum dot, or a core quantum dot, for example. Thequantum dot containing elements belonging to Groups 2 and 16, elementsbelonging to Groups 13 and 15, elements belonging to Groups 13 and 17,elements belonging to Groups 11 and 17, or elements belonging to Groups14 and 15 may be used. Alternatively, the quantum dot containing anelement such as cadmium (Cd), selenium (Se), zinc (Zn), sulfur (S),phosphorus (P), indium (In), tellurium (Te), lead (Pb), gallium (Ga),arsenic (As), or aluminum (Al) may be used.

An example of the liquid medium used for the wet process is an organicsolvent of ketones such as methyl ethyl ketone and cyclohexanone; fattyacid esters such as ethyl acetate; halogenated hydrocarbons such asdichlorobenzene; aromatic hydrocarbons such as toluene, xylene,mesitylene, and cyclohexylbenzene; aliphatic hydrocarbons such ascyclohexane, decalin, and dodecane; dimethylformamide (DMF); dimethylsulfoxide (DMSO); or the like.

Examples of the high molecular compound that can be used for thelight-emitting layer include a phenylenevinylene (PPV) derivative suchas poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene](abbreviation: MEH-PPV) or poly(2,5-dioctyl-1,4-phenylenevinylene); apolyfluorene derivative such as poly(9,9-di-n-octylfluorenyl-2,7-diyl)(abbreviation: PF8),poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazole-4,8-diyl)](abbreviation: F8BT),poly(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(2,2′-bithiophene-5,5′-diyl)](abbreviation: F8T2),poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-(9,10-anthracene)], orpoly[(9,9-dihexylfluorene-2,7-diyl)-alt-(2,5-dimethyl-1,4-phenylene)]; apolyalkylthiophene (PAT) derivative such aspoly(3-hexylthiophen-2,5-diyl) (abbreviation: P3HT); and a polyphenylenederivative. These high molecular compounds, poly(9-vinylcarbazole)(abbreviation: PVK), poly(2-vinylnaphthalene),poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (abbreviation: PTAA), orthe like may be doped with a light-emitting compound and used for thelight-emitting layer. As the light-emitting compound, any of theabove-described light-emitting compounds can be used.

<<Substrate>>

A light-emitting element in one embodiment of the present invention canbe formed over a substrate of glass, plastic, or the like. As the way ofstacking layers over the substrate, layers can be sequentially stackedeither from the electrode 101 side or from the electrode 102 side.

For the substrate over which the light-emitting element of oneembodiment of the present invention can be formed, glass, quartz,plastic, or the like can be used, for example. Alternatively, a flexiblesubstrate can be used. The flexible substrate is a substrate that can bebent, such as a plastic substrate made of polycarbonate or polyarylate,for example. A film, an inorganic film formed by evaporation, or thelike can also be used. Another material may be used as long as thesubstrate functions as a support in a manufacturing process of thelight-emitting element or the optical element. Another material having afunction of protecting the light-emitting element or the optical elementmay be used.

In this specification and the like, a light-emitting element can beformed using any of a variety of substrates, for example. The type of asubstrate is not limited particularly. Examples of the substrate includea semiconductor substrate (e.g., a single crystal substrate or a siliconsubstrate), an SOI substrate, a glass substrate, a quartz substrate, aplastic substrate, a metal substrate, a stainless steel substrate, asubstrate including stainless steel foil, a tungsten substrate, asubstrate including tungsten foil, a flexible substrate, an attachmentfilm, cellulose nanofiber (CNF) and paper which include a fibrousmaterial, a base material film, and the like. As an example of a glasssubstrate, a barium borosilicate glass substrate, an aluminoborosilicateglass substrate, a soda lime glass substrate, or the like can be given.Examples of the flexible substrate, the attachment film, the basematerial film, and the like are substrates of plastics typified bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polyether sulfone (PES), and polytetrafluoroethylene (PTFE). Anotherexample is a resin such as acrylic. Alternatively, polypropylene,polyester, polyvinyl fluoride, polyvinyl chloride, or the like can beused. Alternatively, polyamide, polyimide, aramid, epoxy, an inorganicfilm formed by evaporation, paper, or the like can be used.

Alternatively, a flexible substrate may be used as the substrate, and atransistor or a light-emitting element may be provided directly on theflexible substrate. Still alternatively, a separation layer may beprovided between the substrate and the light-emitting element. Theseparation layer can be used when part or the whole of a light-emittingelement formed over the separation layer is separated from the substrateand transferred onto another substrate. In such a case, thelight-emitting element can be transferred to a substrate having low heatresistance or a flexible substrate as well. For the above separationlayer, a stack including inorganic films, which are a tungsten film anda silicon oxide film, or a structure in which a resin film of polyimideor the like is formed over a substrate can be used, for example.

In other words, after the light-emitting element is formed using asubstrate, the light-emitting element may be transferred to anothersubstrate. Examples of a substrate to which the light-emitting elementis transferred include, in addition to the above-described substrates, acellophane substrate, a stone substrate, a wood substrate, a clothsubstrate (including a natural fiber (e.g., silk, cotton, or hemp), asynthetic fiber (e.g., nylon, polyurethane, or polyester), a regeneratedfiber (e.g., acetate, cupra, rayon, or regenerated polyester), or thelike), a leather substrate, and a rubber substrate. When such asubstrate is used, a light-emitting element with high durability, highheat resistance, reduced weight, or reduced thickness can be formed.

The light-emitting element 150 may be formed over an electrodeelectrically connected to a field-effect transistor (FET), for example,which is formed over any of the above-described substrates. In thatcase, an active matrix display device in which the FET controls thedriving of the light-emitting element can be manufactured.

The structure described above in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 2

In this embodiment, light-emitting elements having structures differentfrom that described in Embodiment 1 and light emission mechanisms of thelight-emitting elements will be described below with reference to FIG.5. In FIG. 5, a portion having a function similar to that in FIG. 1 isrepresented by the same hatch pattern as that in FIG. 1 and notparticularly denoted by a reference numeral in some cases. In addition,common reference numerals are used for portions having similarfunctions, and a detailed description of the portions is omitted in somecases.

<Structure Example of Light-Emitting Element>

FIG. 5 is a schematic cross-sectional view of a light-emitting element250.

The light-emitting element 250 illustrated in FIG. 5 includes aplurality of light-emitting units (a light-emitting unit 106 and alight-emitting unit 108 in FIG. 5) between a pair of electrodes (theelectrode 101 and the electrode 102). One light-emitting unit has thesame structure as the EL layer 100 illustrated in FIG. 1. That is, thelight-emitting element 150 in FIG. 1 includes one light-emitting unit,while the light-emitting element 250 includes a plurality oflight-emitting units. Note that the electrode 101 functions as an anodeand the electrode 102 functions as a cathode in the followingdescription of the light-emitting element 250; however, the functionsmay be interchanged in the light-emitting element 250.

In the light-emitting element 250 illustrated in FIG. 5, thelight-emitting unit 106 and the light-emitting unit 108 are stacked, anda charge-generation layer 115 is provided between the light-emittingunit 106 and the light-emitting unit 108. Note that the structures ofthe light-emitting unit 106 and the light-emitting unit 108 may be thesame or different from each other. For example, it is preferable thatthe light-emitting unit 106 have a structure similar to that of the ELlayer 100 illustrated in FIG. 1.

The light-emitting element 250 includes the light-emitting layer 130 anda light-emitting layer 140. The light-emitting unit 106 includes thehole-injection layer 111, the hole-transport layer 112, anelectron-transport layer 113, and an electron-injection layer 114 inaddition to the light-emitting layer 130. The light-emitting unit 108includes a hole-injection layer 116, a hole-transport layer 117, anelectron-transport layer 118, and the electron-injection layer 119 inaddition to the light-emitting layer 140.

The charge-generation layer 115 may have either a structure in which anacceptor substance that is an electron acceptor is added to ahole-transport material or a structure in which a donor substance thatis an electron donor is added to an electron-transport material.Alternatively, both of these structures may be stacked.

In the case where the charge-generation layer 115 contains a compositematerial of an organic compound and an acceptor substance, the compositematerial that can be used for the hole-injection layer 111 described inEmbodiment 1 may be used for the composite material. As the organiccompound, a variety of compounds such as an aromatic amine compound, acarbazole compound, an aromatic hydrocarbon, and a high molecularcompound (such as an oligomer, a dendrimer, or a polymer) can be used. Asubstance having a hole mobility of 1×10⁻⁶ cm²/Vs or higher ispreferably used as the organic compound. Note that any other materialmay be used as long as it has a property of transporting more holes thanelectrons. Since the composite material of an organic compound and anacceptor substance has excellent carrier-injection and carrier-transportproperties, low-voltage driving or low-current driving can be achieved.Note that when a surface of a light-emitting unit on the anode side isin contact with the charge-generation layer 115 as in the case of thelight-emitting unit 108, the charge-generation layer 115 can also serveas a hole-injection layer or a hole-transport layer of thelight-emitting unit; thus, a hole-injection layer or a hole-transportlayer need not be included in the light-emitting unit.

The charge-generation layer 115 may have a stacked structure of a layercontaining the composite material of an organic compound and an acceptorsubstance and a layer containing another material. For example, thecharge-generation layer 115 may be formed using a combination of a layercontaining the composite material of an organic compound and an acceptorsubstance with a layer containing one compound selected from amongelectron-donating materials and a compound having an excellentelectron-transport property. Furthermore, the charge-generation layer115 may be formed using a combination of a layer containing thecomposite material of an organic compound and an acceptor substance witha layer containing a transparent conductive material.

The charge-generation layer 115 provided between the light-emitting unit106 and the light-emitting unit 108 may have any structure as long aselectrons can be injected into the light-emitting unit on one side andholes can be injected into the light-emitting unit on the other sidewhen a voltage is applied between the electrode 101 and the electrode102. For example, in FIG. 5, the charge-generation layer 115 injectselectrons into the light-emitting unit 106 and holes into thelight-emitting unit 108 when a voltage is applied such that thepotential of the electrode 101 is higher than that of the electrode 102.

Note that in terms of light extraction efficiency, the charge-generationlayer 115 preferably has a visible light transmittance (specifically, avisible light transmittance of higher than or equal to 40%). Thecharge-generation layer 115 functions even if it has lower conductivitythan the pair of electrodes (the electrodes 101 and 102). In the casewhere the conductivity of the charge-generation layer 115 is as high asthose of the pair of electrodes, carriers generated in thecharge-generation layer 115 flow toward the film surface direction, sothat light is emitted in a region where the electrode 101 and theelectrode 102 do not overlap with each other, in some cases. To suppresssuch a defect, the charge-generation layer 115 is preferably formedusing a material whose conductivity is lower than those of the pair ofelectrodes.

Note that forming the charge-generation layer 115 by using any of theabove materials can suppress an increase in drive voltage caused by thestack of the light-emitting layers.

The light-emitting element having two light-emitting units is describedwith reference to FIG. 5; however, a similar structure can be applied toa light-emitting element in which three or more light-emitting units arestacked. With a plurality of light-emitting units partitioned by thecharge-generation layer between a pair of electrodes as in thelight-emitting element 250, it is possible to provide a light-emittingelement which can emit light with high luminance with the currentdensity kept low and has a long lifetime. A light-emitting element withlow power consumption can be provided.

When the structure of the EL layer 100 illustrated in FIG. 1 is used forat least one of the plurality of units, a light-emitting element withhigh luminous efficiency can be provided.

It is preferable that the light-emitting layer 130 included in thelight-emitting unit 106 have the structure described in Embodiment 1, inwhich case the light-emitting element 250 has high luminous efficiency.

Note that the guest materials used in the light-emitting unit 106 andthe light-emitting unit 108 may be the same or different. In the casewhere the same guest material is used for the light-emitting unit 106and the light-emitting unit 108, the light-emitting element 250 canexhibit high emission luminance at a small current value, which ispreferable. In the case where different guest materials are used for thelight-emitting unit 106 and the light-emitting unit 108, thelight-emitting element 250 can exhibit multi-color light emission, whichis preferable. It is particularly favorable to select the guestmaterials so that white light emission with high color renderingproperties or light emission of at least red, green, and blue can beobtained.

Note that the light-emitting units 106 and 108 and the charge-generationlayer 115 can be formed by an evaporation method (including a vacuumevaporation method), an ink-jet method, a coating method, gravureprinting, or the like.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 3

In this embodiment, examples of light-emitting elements havingstructures different from those described in Embodiments 1 and 2 will bedescribed below with reference to FIG. 6 and FIGS. 7A and 7B.

<Structure Example 1 of Light-Emitting Element>

FIG. 6 is a cross-sectional view illustrating a light-emitting elementof one embodiment of the present invention. In FIG. 6, a portion havinga function similar to that in FIG. 1 is represented by the same hatchpattern as that in FIG. 1 and not particularly denoted by a referencenumeral in some cases. In addition, common reference numerals are usedfor portions having similar functions, and a detailed description of theportions is omitted in some cases.

A light-emitting element 260 in FIG. 6 may have a bottom-emissionstructure in which light is extracted through a substrate 200 or mayhave a top-emission structure in which light is extracted in thedirection opposite to the substrate 200. However, one embodiment of thepresent invention is not limited to this structure, and a light-emittingelement having a dual-emission structure in which light emitted from thelight-emitting element is extracted in both top and bottom directions ofthe substrate 200 may be used.

In the case where the light-emitting element 260 has a bottom emissionstructure, the electrode 101 preferably has a function of transmittinglight and the electrode 102 preferably has a function of reflectinglight. Alternatively, in the case where the light-emitting element 260has a top emission structure, the electrode 101 preferably has afunction of reflecting light and the electrode 102 preferably has afunction of transmitting light.

The light-emitting element 260 includes the electrode 101 and theelectrode 102 over the substrate 200. Between the electrodes 101 and102, a light-emitting layer 123B, a light-emitting layer 123G, and alight-emitting layer 123R are provided. The hole-injection layer 111,the hole-transport layer 112, the electron-transport layer 118, and theelectron-injection layer 119 are also provided.

The electrode 101 may be formed using a plurality of conductive layers.In that case, it is preferable that a conductive layer having a functionof reflecting light and a conductive layer having a function oftransmitting light be stacked.

For the electrode 101, a structure and materials similar to those of theelectrode 101 or 102 described in Embodiment 1 can be used.

In FIG. 6, a partition 145 is provided between a region 221B, a region221G, and a region 221R, which are sandwiched between the electrode 101and the electrode 102. The partition 145 has an insulating property. Thepartition 145 covers end portions of the electrode 101 and has openingsoverlapping with the electrode. With the partition 145, the electrode101 provided over the substrate 200 in the regions can be divided intoisland shapes.

Note that the light-emitting layer 123B and the light-emitting layer123G may overlap with each other in a region where they overlap with thepartition 145. The light-emitting layer 123G and the light-emittinglayer 123R may overlap with each other in a region where they overlapwith the partition 145. The light-emitting layer 123R and thelight-emitting layer 123B may overlap with each other in a region wherethey overlap with the partition 145.

The partition 145 has an insulating property and is formed using aninorganic or organic material. Examples of the inorganic materialinclude silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, and aluminum nitride. Examples of theorganic material include photosensitive resin materials such as anacrylic resin and a polyimide resin.

The light-emitting layers 123R, 123G, and 123B preferably containlight-emitting materials having functions of emitting light of differentcolors. For example, when the light-emitting layer 123R contains alight-emitting material having a function of emitting red light, theregion 221R emits red light. When the light-emitting layer 123G containsa light-emitting material having a function of emitting green light, theregion 221G emits green light. When the light-emitting layer 123Bcontains a light-emitting material having a function of emitting bluelight, the region 221B emits blue light. The light-emitting element 260having such a structure is used in a pixel of a display device, wherebya full-color display device can be fabricated. The thicknesses of thelight-emitting layers may be the same or different.

One or more of the light-emitting layer 123B, the light-emitting layer123G, and the light-emitting layer 123R preferably have a structuresimilar to that of the light-emitting layer 130 described inEmbodiment 1. In that case, a light-emitting element with high luminousefficiency can be fabricated.

One or more of the light-emitting layers 123B, 123G, and 123R mayinclude two or more stacked layers.

When at least one light-emitting layer includes the light-emitting layerdescribed in Embodiment 1 as described above and the light-emittingelement 260 including the light-emitting layer is used in pixels in adisplay device, a display device with high luminous efficiency can befabricated. The display device including the light-emitting element 260can thus have reduced power consumption.

By providing a color filter over the electrode through which light isextracted, the color purity of the light-emitting element 260 can beimproved. Therefore, the color purity of a display device including thelight-emitting element 260 can be improved.

By providing a polarizing plate over the electrode through which lightis extracted, the reflection of external light by the light-emittingelement 260 can be reduced. Therefore, the contrast ratio of a displaydevice including the light-emitting element 260 can be improved.

For the other components of the light-emitting element 260, thecomponents of the light-emitting element in Embodiment 1 can be referredto.

<Structure Example 2 of Light-Emitting Element>

Next, structural examples different from the light-emitting elementillustrated in FIG. 6 will be described below with reference to FIGS. 7Aand 7B.

FIGS. 7A and 7B are cross-sectional views of a light-emitting element ofone embodiment of the present invention. In FIGS. 7A and 7B, a portionhaving a function similar to that in FIG. 6 is represented by the samehatch pattern as that in FIG. 6 and not particularly denoted by areference numeral in some cases. In addition, common reference numeralsare used for portions having similar functions, and a detaileddescription of such portions is not repeated in some cases.

FIGS. 7A and 7B illustrate structural examples of a light-emittingelement including the light-emitting layer between a pair of electrodes.A light-emitting element 262 a illustrated in FIG. 7A has a top-emissionstructure in which light is extracted in a direction opposite to thesubstrate 200, and a light-emitting element 262 b illustrated in FIG. 7Bhas a bottom-emission structure in which light is extracted to thesubstrate 200 side. However, one embodiment of the present invention isnot limited to these structures and may have a dual-emission structurein which light emitted from the light-emitting element is extracted inboth top and bottom directions with respect to the substrate 200 overwhich the light-emitting element is formed.

The light-emitting elements 262 a and 262 b each include the electrode101, the electrode 102, an electrode 103, and an electrode 104 over thesubstrate 200. At least a light-emitting layer 130 and thecharge-generation layer 115 are provided between the electrode 101 andthe electrode 102, between the electrode 102 and the electrode 103, andbetween the electrode 102 and the electrode 104. The hole-injectionlayer 111, the hole-transport layer 112, the light-emitting layer 140,the electron-transport layer 113, the electron-injection layer 114, thehole-injection layer 116, the hole-transport layer 117, theelectron-transport layer 118, and the electron-injection layer 119 arefurther provided.

The electrode 101 includes a conductive layer 101 a and a conductivelayer 101 b over and in contact with the conductive layer 101 a. Theelectrode 103 includes a conductive layer 103 a and a conductive layer103 b over and in contact with the conductive layer 103 a. The electrode104 includes a conductive layer 104 a and a conductive layer 104 b overand in contact with the conductive layer 104 a.

The light-emitting element 262 a illustrated in FIG. 7A and thelight-emitting element 262 b illustrated in FIG. 7B each include thepartition 145 between a region 222B sandwiched between the electrode 101and the electrode 102, a region 222G sandwiched between the electrode102 and the electrode 103, and a region 222R sandwiched between theelectrode 102 and the electrode 104. The partition 145 has an insulatingproperty. The partition 145 covers end portions of the electrodes 101,103, and 104 and has openings overlapping with the electrodes. With thepartition 145, the electrodes provided over the substrate 200 in theregions can be separated into island shapes.

The light-emitting elements 262 a and 262 b each include a substrate 220provided with an optical element 224B, an optical element 224G, and anoptical element 224R in the direction in which light emitted from theregion 222B, light emitted from the region 222G, and light emitted fromthe region 222R are extracted. The light emitted from each region isemitted outside the light-emitting element through each optical element.In other words, the light from the region 222B, the light from theregion 222G, and the light from the region 222R are emitted through theoptical element 224B, the optical element 224G, and the optical element224R, respectively.

The optical elements 224B, 224G, and 224R each have a function ofselectively transmitting light of a particular color out of incidentlight. For example, the light emitted from the region 222B through theoptical element 224B is blue light, the light emitted from the region222G through the optical element 224G is green light, and the lightemitted from the region 222R through the optical element 224R is redlight.

For example, a coloring layer (also referred to as color filter), a bandpass filter, a multilayer filter, or the like can be used for theoptical elements 224R, 224G, and 224B. Alternatively, color conversionelements can be used as the optical elements. A color conversion elementis an optical element that converts incident light into light having alonger wavelength than the incident light. As the color conversionelements, quantum-dot elements can be favorably used. The use of thequantum-dot type can increase color reproducibility of the displaydevice.

A plurality of optical elements may also be stacked over each of theoptical elements 224R, 224G, and 224B. As another optical element, acircularly polarizing plate, an anti-reflective film, or the like can beprovided, for example. A circularly polarizing plate provided on theside where light emitted from the light-emitting element of the displaydevice is extracted can prevent a phenomenon in which light incidentfrom the outside of the display device is reflected inside the displaydevice and returned to the outside. An anti-reflective film can weakenexternal light reflected by a surface of the display device. This leadsto clear observation of light emitted from the display device.

Note that in FIGS. 7A and 7B, blue light (B), green light (G), and redlight (R) emitted from the regions through the optical elements areschematically illustrated by arrows of dashed lines.

A light-blocking layer 223 is provided between the optical elements. Thelight-blocking layer 223 has a function of blocking light emitted fromthe adjacent regions. Note that a structure without the light-blockinglayer 223 may also be employed.

The light-blocking layer 223 has a function of reducing the reflectionof external light. The light-blocking layer 223 has a function ofpreventing mixture of light emitted from an adjacent light-emittingelement. For the light-blocking layer 223, a metal, a resin containingblack pigment, carbon black, a metal oxide, a composite oxide containinga solid solution of a plurality of metal oxides, or the like can beused.

For the substrate 200 and the substrate 220 provided with the opticalelements, the substrate in Embodiment 1 can be referred to.

Furthermore, the light-emitting elements 262 a and 262 b have amicrocavity structure.

<<Microcavity Structure>>

Light emitted from the light-emitting layer 130 and the light-emittinglayer 140 resonates between a pair of electrodes (e.g., the electrode101 and the electrode 102). The light-emitting layer 130 and thelight-emitting layer 140 are formed at such a position as to intensifythe light of a desired wavelength among light to be emitted. Forexample, by adjusting the optical length from a reflective region of theelectrode 101 to the light-emitting region of the light-emitting layer130 and the optical length from a reflective region of the electrode 102to the light-emitting region of the light-emitting layer 130, the lightof a desired wavelength among light emitted from the light-emittinglayer 130 can be intensified. By adjusting the optical length from thereflective region of the electrode 101 to the light-emitting region ofthe light-emitting layer 140 and the optical length from the reflectiveregion of the electrode 102 to the light-emitting region of thelight-emitting layer 140, the light of a desired wavelength among lightemitted from the light-emitting layer 140 can be intensified. In thecase of a light-emitting element in which a plurality of light-emittinglayers (here, the light-emitting layers 130 and 140) are stacked, theoptical lengths of the light-emitting layers 130 and 140 are preferablyoptimized.

In each of the light-emitting elements 262 a and 262 b, by adjusting thethicknesses of the conductive layers (the conductive layer 101 b, theconductive layer 103 b, and the conductive layer 104 b) in each region,the light of a desired wavelength among light emitted from thelight-emitting layers 130 and 140 can be intensified. Note that thethickness of at least one of the hole-injection layer 111 and thehole-transport layer 112 may differ between the regions to intensify thelight emitted from the light-emitting layers 130 and 140.

For example, in the case where the refractive index of the conductivematerial having a function of reflecting light in the electrodes 101 to104 is lower than the refractive index of the light-emitting layer 130or 140, the thickness of the conductive layer 101 b of the electrode 101is adjusted so that the optical length between the electrode 101 and theelectrode 102 is m_(B)λ_(B)/2 (m_(B) is a natural number and λ_(B) isthe wavelength of light intensified in the region 222B). Similarly, thethickness of the conductive layer 103 b of the electrode 103 is adjustedso that the optical length between the electrode 103 and the electrode102 is m_(G)λ_(G)/2 (m_(G) is a natural number and λ_(G) is thewavelength of light intensified in the region 222G). Furthermore, thethickness of the conductive layer 104 b of the electrode 104 is adjustedso that the optical length between the electrode 104 and the electrode102 is m_(R)λ_(R)/2 (m_(R) is a natural number and λ_(R) is thewavelength of light intensified in the region 222R).

In the above manner, with the microcavity structure, in which theoptical length between the pair of electrodes in the respective regionsis adjusted, scattering and absorption of light in the vicinity of theelectrodes can be suppressed, resulting in high light extractionefficiency. In the above structure, the conductive layers 101 b, 103 b,and 104 b preferably have a function of transmitting light. Thematerials of the conductive layers 101 b, 103 b, and 104 b may be thesame or different. Each of the conductive layers 101 b, 103 b, and 104 bmay have a stacked structure of two or more layers.

Since the light-emitting element 262 a illustrated in FIG. 7A has atop-emission structure, it is preferable that the conductive layer 101a, the conductive layer 103 a, and the conductive layer 104 a have afunction of reflecting light. In addition, it is preferable that theelectrode 102 have functions of transmitting light and reflecting light.

Since the light-emitting element 262 b illustrated in FIG. 7B has abottom-emission structure, it is preferable that the conductive layer101 a, the conductive layer 103 a, and the conductive layer 104 a havefunctions of transmitting light and reflecting light. In addition, it ispreferable that the electrode 102 have a function of reflecting light.

In each of the light-emitting elements 262 a and 262 b, the conductivelayers 101 a, 103 a, and 104 a may be formed of different materials orthe same material. When the conductive layers 101 a, 103 a, and 104 aare formed of the same material, manufacturing cost of thelight-emitting elements 262 a and 262 b can be reduced. Note that eachof the conductive layers 101 a, 103 a, and 104 a may have a stackedstructure including two or more layers.

The light-emitting layer 130 in the light-emitting elements 262 a and262 b preferably has the structure described in Embodiment 1, in whichcase light-emitting elements with high luminous efficiency can befabricated.

Either or both of the light-emitting layers 130 and 140 may have astacked structure of two layers, like a light-emitting layer 140 a and alight-emitting layer 140 b. The two light-emitting layers including twokinds of light-emitting materials (a first light-emitting material and asecond light-emitting material) for emitting different colors of lightenable light emission of a plurality of colors. It is particularlypreferable to select the light-emitting materials of the light-emittinglayers so that white light can be obtained by combining light emissionsfrom the light-emitting layers 130 and 140.

Either or both of the light-emitting layers 130 and 140 may have astacked structure of three or more layers, in which a layer notincluding a light-emitting material may be included.

In the above-described manner, the light-emitting element 262 a or 262 bincluding the light-emitting layer which has the structure described inEmbodiment 1 is used in pixels in a display device, whereby a displaydevice with high luminous efficiency can be fabricated. Accordingly, thedisplay device including the light-emitting element 262 a or 262 b canhave low power consumption.

For the other components of the light-emitting elements 262 a and 262 b,the components of the light-emitting element 260 or the light-emittingelement in Embodiment 1 or 2 can be referred to.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 4

In this embodiment, a display device including a light-emitting elementof one embodiment of the present invention will be described withreference to FIGS. 8A and 8B, FIGS. 9A and 9B, and FIGS. 10A and 10B.

<Structure Example 1 of Display Device>

FIG. 8A is a top view illustrating a display device 600 and FIG. 8B is across-sectional view taken along the dashed-dotted line A-B and thedashed-dotted line C-D in FIG. 8A. The display device 600 includesdriver circuit portions (a signal line driver circuit portion 601 and ascan line driver circuit portion 603) and a pixel portion 602. Note thatthe signal line driver circuit portion 601, the scan line driver circuitportion 603, and the pixel portion 602 have a function of controllinglight emission from a light-emitting element.

The display device 600 also includes an element substrate 610, a sealingsubstrate 604, a sealant 605, a region 607 surrounded by the sealant605, a lead wiring 608, and an FPC 609.

Note that the lead wiring 608 is a wiring for transmitting signals to beinput to the signal line driver circuit portion 601 and the scan linedriver circuit portion 603 and for receiving a video signal, a clocksignal, a start signal, a reset signal, and the like from the FPC 609serving as an external input terminal. Although only the FPC 609 isillustrated here, the FPC 609 may be provided with a printed wiringboard (PWB).

As the signal line driver circuit portion 601, a CMOS circuit in whichan n-channel transistor 623 and a p-channel transistor 624 are combinedis formed. As the signal line driver circuit portion 601 or the scanline driver circuit portion 603, various types of circuits such as aCMOS circuit, a PMOS circuit, or an NMOS circuit can be used. Although adriver in which a driver circuit portion is formed and a pixel areformed over the same surface of a substrate in the display device ofthis embodiment, the driver circuit portion is not necessarily formedover the substrate and can be formed outside the substrate.

The pixel portion 602 includes a switching transistor 611, a currentcontrol transistor 612, and a lower electrode 613 electrically connectedto a drain of the current control transistor 612. Note that a partitionwall 614 is formed to cover end portions of the lower electrode 613. Asthe partition wall 614, for example, a positive type photosensitiveacrylic resin film can be used.

In order to obtain favorable coverage, the partition wall 614 is formedto have a curved surface with curvature at its upper or lower endportion. For example, in the case of using a positive photosensitiveacrylic as a material of the partition wall 614, it is preferable thatonly the upper end portion of the partition wall 614 have a curvedsurface with curvature (the radius of the curvature being 0.2 μm to 3μm). As the partition wall 614, either a negative photosensitive resinor a positive photosensitive resin can be used.

Note that there is no particular limitation on a structure of each ofthe transistors (the transistors 611, 612, 623, and 624). For example, astaggered transistor can be used. In addition, there is no particularlimitation on the polarity of these transistors. For these transistors,n-channel and p-channel transistors may be used, or either n-channeltransistors or p-channel transistors may be used, for example.Furthermore, there is no particular limitation on the crystallinity of asemiconductor film used for the transistors. For example, an amorphoussemiconductor film or a crystalline semiconductor film may be used.Examples of a semiconductor material include Group 14 semiconductors(e.g., a semiconductor including silicon), compound semiconductors(including oxide semiconductors), organic semiconductors, and the like.For example, it is preferable to use an oxide semiconductor that has anenergy gap of 2 eV or more, preferably 2.5 eV or more, and furtherpreferably 3 eV or more, for the transistors, so that the off-statecurrent of the transistors can be reduced. Examples of the oxidesemiconductor include an In—Ga oxide, an In-M-Zn oxide (M is aluminum(Al), gallium (Ga), yttrium (Y), zirconium (Zr), lanthanum (La), cerium(Ce), tin (Sn), hafnium (Hf), or neodymium (Nd)), and the like.

An EL layer 616 and an upper electrode 617 are formed over the lowerelectrode 613. Here, the lower electrode 613 functions as an anode andthe upper electrode 617 functions as a cathode.

In addition, the EL layer 616 is formed by any of various methodsincluding an evaporation method (including a vacuum evaporation method)with an evaporation mask, a droplet discharge method (also referred toas an ink-jet method), a coating method such as a spin coating method,and a gravure printing method. As another material included in the ELlayer 616, a low molecular compound or a high molecular compound(including an oligomer or a dendrimer) may be used.

Note that a light-emitting element 618 is formed with the lowerelectrode 613, the EL layer 616, and the upper electrode 617. Thelight-emitting element 618 preferably has any of the structuresdescribed in Embodiments 1 to 3. In the case where the pixel portionincludes a plurality of light-emitting elements, the pixel portion mayinclude both any of the light-emitting elements described in Embodiments1 to 3 and a light-emitting element having a different structure.

When the sealing substrate 604 and the element substrate 610 areattached to each other with the sealant 605, the light-emitting element618 is provided in the region 607 surrounded by the element substrate610, the sealing substrate 604, and the sealant 605. The region 607 isfilled with a filler. In some cases, the region 607 is filled with aninert gas (nitrogen, argon, or the like) or filled with an ultravioletcurable resin or a thermosetting resin which can be used for the sealant605. For example, a polyvinyl chloride (PVC) based resin, an acrylicresin, a polyimide-based resin, an epoxy-based resin, a silicone-basedresin, a polyvinyl butyral (PVB) based resin, or an ethylene vinylacetate (EVA) based resin can be used. It is preferable that the sealingsubstrate be provided with a recessed portion and a desiccant beprovided in the recessed portion, in which case deterioration due toinfluence of moisture can be inhibited.

An optical element 621 is provided below the sealing substrate 604 tooverlap with the light-emitting element 618. A light-blocking layer 622is provided below the sealing substrate 604. The structures of theoptical element 621 and the light-blocking layer 622 can be the same asthose of the optical element and the light-blocking layer in Embodiment3, respectively.

An epoxy-based resin or glass frit is preferably used for the sealant605. It is preferable that such a material do not transmit moisture oroxygen as much as possible. As the sealing substrate 604, a glasssubstrate, a quartz substrate, or a plastic substrate formed of fiberreinforced plastics (FRP), poly(vinyl fluoride) (PVF), polyester,acrylic, or the like can be used.

In the above-described manner, a display device including any of thelight-emitting elements and the optical elements which are described inEmbodiments 1 to 3 can be obtained.

<Structure Example 2 of Display Device>

Next, another example of the display device will be described withreference to FIGS. 9A and 9B. Note that FIGS. 9A and 9B are each across-sectional view of a display device of one embodiment of thepresent invention.

In FIG. 9A, a substrate 1001, a base insulating film 1002, a gateinsulating film 1003, gate electrodes 1006, 1007, and 1008, a firstinterlayer insulating film 1020, a second interlayer insulating film1021, a peripheral portion 1042, a pixel portion 1040, a driver circuitportion 1041, lower electrodes 1024R, 1024G, and 1024B of light-emittingelements, a partition wall 1025, an EL layer 1028, an upper electrode1026 of the light-emitting elements, a sealing layer 1029, a sealingsubstrate 1031, a sealant 1032, and the like are illustrated.

In FIG. 9A, examples of the optical elements, coloring layers (a redcoloring layer 1034R, a green coloring layer 1034G, and a blue coloringlayer 1034B) are provided on a transparent base material 1033. Further,a light-blocking layer 1035 may be provided. The transparent basematerial 1033 provided with the coloring layers and the light-blockinglayer is positioned and fixed to the substrate 1001. Note that thecoloring layers and the light-blocking layer are covered with anovercoat layer 1036. In the structure in FIG. 9A, the coloring layerstransmit red light, green light, and blue light, and thus an image canbe displayed with the use of pixels of three colors.

FIG. 9B illustrates an example in which, as examples of the opticalelements, the coloring layers (the red coloring layer 1034R, the greencoloring layer 1034G, and the blue coloring layer 1034B) are providedbetween the gate insulating film 1003 and the first interlayerinsulating film 1020. As in this structure, the coloring layers may beprovided between the substrate 1001 and the sealing substrate 1031.

As examples of the optical elements, the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) are provided between the first interlayerinsulating film 1020 and the second interlayer insulating film 1021.

The above-described display device has a structure in which light isextracted from the substrate 1001 side where the transistors are formed(a bottom-emission structure), but may have a structure in which lightis extracted from the sealing substrate 1031 side (a top-emissionstructure).

<Structure Example 3 of Display Device>

FIGS. 10A and 10B are each an example of a cross-sectional view of adisplay device having a top-emission structure. Note that FIGS. 10A and10B are each a cross-sectional view illustrating the display device ofone embodiment of the present invention, and the driver circuit portion1041, the peripheral portion 1042, and the like, which are illustratedin FIGS. 9A and 9B, are not illustrated therein.

In that case, a substrate which does not transmit light can be used asthe substrate 1001. The process up to the step of forming a connectionelectrode which connects the transistor and the anode of thelight-emitting element is performed in a manner similar to that of thedisplay device having a bottom-emission structure. Then, a thirdinterlayer insulating film 1037 is formed to cover an electrode 1022.This insulating film may have a planarization function. The thirdinterlayer insulating film 1037 can be formed using a material similarto that of the second interlayer insulating film, or can be formed usingany of various other materials.

The lower electrodes 1024R, 1024G, and 1024B of the light-emittingelements each function as an anode here, but may function as a cathode.Further, in the case of a display device having a top-emission structureas illustrated in FIGS. 10A and 10B, the lower electrodes 1024R, 1024G,and 1024B preferably have a function of reflecting light. The upperelectrode 1026 is provided over the EL layer 1028. It is preferable thatthe upper electrode 1026 have a function of reflecting light and afunction of transmitting light and that a microcavity structure be usedbetween the upper electrode 1026 and the lower electrodes 1024R, 1024G,and 1024B, in which case the intensity of light having a specificwavelength is increased.

In the case of a top-emission structure as illustrated in FIG. 10A,sealing can be performed with the sealing substrate 1031 on which thecoloring layers (the red coloring layer 1034R, the green coloring layer1034G, and the blue coloring layer 1034B) are provided. The sealingsubstrate 1031 may be provided with the light-blocking layer 1035 whichis positioned between pixels. Note that a light-transmitting substrateis favorably used as the sealing substrate 1031.

FIG. 10A illustrates the structure provided with the light-emittingelements and the coloring layers for the light-emitting elements as anexample; however, the structure is not limited thereto. For example, asshown in FIG. 10B, a structure including the red coloring layer 1034Rand the blue coloring layer 1034B but not including a green coloringlayer may be employed to achieve full color display with the threecolors of red, green, and blue. The structure as illustrated in FIG. 10Awhere the light-emitting elements are provided with the coloring layersis effective to suppress reflection of external light. In contrast, thestructure as illustrated in FIG. 10B where the light-emitting elementsare provided with the red coloring layer and the blue coloring layer butnot with the green coloring layer is effective in reducing powerconsumption because of small energy loss of light emitted from thegreen-light-emitting element.

Although a display device including sub-pixels of three colors (red,green, and blue) is described above, the number of colors of sub-pixelsmay be four (red, green, blue, and yellow, or red, green, blue, andwhite). In this case, a coloring layer can be used which has a functionof transmitting yellow light or a function of transmitting light of aplurality of colors selected from blue, green, yellow, and red. In thecase where the coloring layer can transmit light of a plurality ofcolors selected from blue, green, yellow, and red, light passing throughthe coloring layer may be white light. Since the light-emitting elementwhich exhibits yellow or white light has high luminous efficiency, thedisplay device having such a structure can have lower power consumption.

Furthermore, in the display device 600 shown in FIGS. 8A and 8B, asealing layer may be formed in the region 607 which is surrounded by theelement substrate 610, the sealing substrate 604, and the sealant 605.For the sealing layer, a resin such as a polyvinyl chloride (PVC) basedresin, an acrylic resin, a polyimide-based resin, an epoxy-based resin,a silicone-based resin, a polyvinyl butyral (PVB) based resin, or anethylene vinyl acetate (EVA) based resin can be used. Alternatively, aninorganic material such as silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, aluminum oxide, or aluminum nitride canbe used. The formation of the sealing layer in the region 607 canprevent deterioration of the light-emitting element 618 due toimpurities such as water, which is preferable. Note that in the casewhere the sealing layer is formed, the sealant 605 is not necessarilyprovided.

When the sealing layer has a multilayer structure, the impurities suchas water can be effectively prevented from entering the light-emittingelement 618 which is inside the display device from the outside of thedisplay device 600. In the case where the sealing layer has a multilayerstructure, a resin and an inorganic material are preferably stacked.

The structures described in this embodiment can be combined asappropriate with any of the other structures in this embodiment and theother embodiments.

Embodiment 5

In this embodiment, a display module, electronic devices, alight-emitting device, and lighting devices each including thelight-emitting element of one embodiment of the present invention aredescribed with reference to FIG. 11, FIGS. 12A to 12G, FIGS. 13A to 13C,and FIG. 14.

<Display Module>

In a display module 8000 in FIG. 11, a touch sensor 8004 connected to anFPC 8003, a display device 8006 connected to an FPC 8005, a frame 8009,a printed board 8010, and a battery 8011 are provided between an uppercover 8001 and a lower cover 8002.

The light-emitting element of one embodiment of the present inventioncan be used for the display device 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate depending on the sizes of the touch sensor8004 and the display device 8006.

The touch sensor 8004 can be a resistive touch sensor or a capacitivetouch sensor and may be formed to overlap with the display device 8006.A counter substrate (sealing substrate) of the display device 8006 canhave a touch sensor function. A photosensor may be provided in eachpixel of the display device 8006 so that an optical touch sensor isobtained.

The frame 8009 protects the display device 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 can alsofunction as a radiator plate.

The printed board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 8011 provided separatelymay be used. The battery 8011 can be omitted in the case of using acommercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<Electronic Device>

FIGS. 12A to 12G show electronic devices. These electronic devices caneach include a housing 9000, a display portion 9001, a speaker 9003,operation keys 9005 (including a power switch or an operation switch), aconnection terminal 9006, a sensor 9007 (a sensor having a function ofmeasuring or sensing force, displacement, position, speed, acceleration,angular velocity, rotational frequency, distance, light, liquid,magnetism, temperature, chemical substance, sound, time, hardness,electric field, current, voltage, electric power, radiation, flow rate,humidity, gradient, oscillation, odor, or infrared ray), a microphone9008, and the like. In addition, the sensor 9007 may have a function ofmeasuring biological information like a pulse sensor and a finger printsensor.

The electronic devices illustrated in FIGS. 12A to 12G can have avariety of functions, for example, a function of displaying a variety ofdata (a still image, a moving image, a text image, and the like) on thedisplay portion, a touch sensor function, a function of displaying acalendar, date, time, and the like, a function of controlling a processwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Note that the electronic devices illustrated inFIGS. 12A to 12G can have a variety of functions without limitation tothe above functions. Although not illustrated in FIGS. 12A to 12G, theelectronic devices may include a plurality of display portions. Theelectronic devices may have a camera or the like and a function oftaking a still image, a function of taking a moving image, a function ofstoring the taken image in a memory medium (an external memory medium ora memory medium incorporated in the camera), a function of displayingthe taken image on the display portion, or the like.

The electronic devices in FIGS. 12A to 12G will be described in detailbelow.

FIG. 12A is a perspective view of a portable information terminal 9100.The display portion 9001 of the portable information terminal 9100 isflexible. Therefore, the display portion 9001 can be incorporated alonga curved surface of a curved housing 9000. In addition, the displayportion 9001 includes a touch sensor, and operation can be performed bytouching the screen with a finger, a stylus, or the like. For example,when an icon displayed on the display portion 9001 is touched, anapplication can be started.

FIG. 12B is a perspective view illustrating a portable informationterminal 9101. The portable information terminal 9101 functions as, forexample, one or more of a telephone set, a notebook, and an informationbrowsing system. Specifically, the portable information terminal can beused as a smartphone. Note that the speaker 9003, the connectionterminal 9006, the sensor 9007, and the like, which are not illustratedin FIG. 12A, can be positioned in the portable information terminal 9101as in the portable information terminal 9100 illustrated in FIG. 12A.The portable information terminal 9101 can display characters and imageinformation on its plurality of surfaces. For example, three operationbuttons 9050 (also referred to as operation icons, or simply, icons) canbe displayed on one surface of the display portion 9001. Furthermore,information 9051 indicated by dashed rectangles can be displayed onanother surface of the display portion 9001. Examples of the information9051 include display indicating reception of an incoming email, socialnetworking service (SNS) message, call, and the like; the title andsender of an email and SNS message; the date; the time; remainingbattery; and display indicating the strength of a received signal suchas a radio wave. Instead of the information 9051, the operation buttons9050 or the like may be displayed on the position where the information9051 is displayed.

As a material of the housing 9000, for example, an alloy, plastic, orceramic can be used. As a plastic, a reinforced plastic can also beused. A carbon fiber reinforced plastics (CFRP), which is a kind ofreinforced plastic, has advantages of being lightweight andcorrosion-free. Other examples of the reinforced plastic include oneincluding glass fiber and one including aramid fiber. As the alloy, analuminum alloy and a magnesium alloy can be given. In particular,amorphous alloy (also referred to as metal glass) containing zirconium,copper, nickel, and titanium is superior in terms of high elasticstrength. This amorphous alloy includes a glass transition region atroom temperature, which is also referred to as a bulk-solidifyingamorphous alloy and substantially has an amorphous atomic structure. Bya solidification casting method, an alloy material is put in a mold ofat least part of the housing and coagulated so that the part of thehousing is formed using a bulk-solidifying amorphous alloy. Theamorphous alloy may include beryllium, silicon, niobium, boron, gallium,molybdenum, tungsten, manganese, iron, cobalt, yttrium, vanadium,phosphorus, carbon, or the like in addition to zirconium, copper,nickel, and titanium. The amorphous alloy may be formed by a vacuumevaporation method, a sputtering method, an electroplating method, anelectroless plating method, or the like instead of the solidificationcasting method. The amorphous alloy may include a microcrystal or ananocrystal as long as a state without a long-range order (a periodicstructure) is maintained as a whole. Note that the term alloy refer toboth a complete solid solution alloy which has a single solid phasestructure and a partial solution that has two or more phases. Thehousing 9000 using the amorphous alloy can have high elastic strength.Even if the portable information terminal 9101 is dropped and the impactcauses temporary deformation, the use of the amorphous alloy in thehousing 9000 allows a return to the original shape; thus, the impactresistance of the portable information terminal 9101 can be improved.

FIG. 12C is a perspective view illustrating a portable informationterminal 9102. The portable information terminal 9102 has a function ofdisplaying information on three or more surfaces of the display portion9001. Here, information 9052, information 9053, and information 9054 aredisplayed on different surfaces. For example, a user of the portableinformation terminal 9102 can see the display (here, the information9053) with the portable information terminal 9102 put in a breast pocketof his/her clothes. Specifically, a caller's phone number, name, or thelike of an incoming call is displayed in a position that can be seenfrom above the portable information terminal 9102. Thus, the user cansee the display without taking out the portable information terminal9102 from the pocket and decide whether to answer the call.

FIG. 12D is a perspective view of a watch-type portable informationterminal 9200. The portable information terminal 9200 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, viewing and editing texts, music reproduction, Internetcommunication, and computer games. The display surface of the displayportion 9001 is bent, and images can be displayed on the bent displaysurface. The portable information terminal 9200 can employ near fieldcommunication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible. The portable information terminal 9200 includes the connectionterminal 9006, and data can be directly transmitted to and received fromanother information terminal via a connector. Power charging through theconnection terminal 9006 is possible. Note that the charging operationmay be performed by wireless power feeding without using the connectionterminal 9006.

FIGS. 12E, 12F, and 12G are perspective views of a foldable portableinformation terminal 9201. FIG. 12E is a perspective view illustratingthe portable information terminal 9201 that is opened. FIG. 12F is aperspective view illustrating the portable information terminal 9201that is shifted from the opened state to the folded state or from thefolded state to the opened state. FIG. 12G is a perspective viewillustrating the portable information terminal 9201 that is folded. Theportable information terminal 9201 is highly portable when folded. Whenthe portable information terminal 9201 is opened, a seamless largedisplay region is highly browsable. The display portion 9001 of theportable information terminal 9201 is supported by three housings 9000joined together by hinges 9055. By folding the portable informationterminal 9201 at a connection portion between two housings 9000 with thehinges 9055, the portable information terminal 9201 can be reversiblychanged in shape from the opened state to the folded state. For example,the portable information terminal 9201 can be bent with a radius ofcurvature of greater than or equal to 1 mm and less than or equal to 150mm.

Examples of electronic devices are a television set (also referred to asa television or a television receiver), a monitor of a computer or thelike, a digital camera, a digital video camera, a digital photo frame, amobile phone handset (also referred to as a mobile phone or a mobilephone device), a goggle-type display (head mounted display), a portablegame machine, a portable information terminal, an audio reproducingdevice, and a large-sized game machine such as a pachinko machine.

Furthermore, the electronic device of one embodiment of the presentinvention may include a secondary battery. It is preferable that thesecondary battery be capable of being charged by non-contact powertransmission.

Examples of the secondary battery include a lithium ion secondarybattery such as a lithium polymer battery using a gel electrolyte(lithium ion polymer battery), a lithium-ion battery, a nickel-hydridebattery, a nickel-cadmium battery, an organic radical battery, alead-acid battery, an air secondary battery, a nickel-zinc battery, anda silver-zinc battery.

The electronic device of one embodiment of the present invention mayinclude an antenna. When a signal is received by the antenna, theelectronic device can display an image, data, or the like on a displayportion. When the electronic device includes a secondary battery, theantenna may be used for non-contact power transmission.

The electronic device or the lighting device of one embodiment of thepresent invention has flexibility and therefore can be incorporatedalong a curved inside/outside wall surface of a house or a building or acurved interior/exterior surface of a car. For example, the electronicdevice or the lighting device can be used for lighting for a dashboard,a windshield, a ceiling, and the like of a car.

<Light-Emitting Device>

FIG. 13A is a perspective view of a light-emitting device 3000 shown inthis embodiment, and FIG. 13B is a cross-sectional view alongdashed-dotted line E-F in FIG. 13A. Note that in FIG. 13A, somecomponents are illustrated by broken lines in order to avoid complexityof the drawing.

The light-emitting device 3000 illustrated in FIGS. 13A and 13B includesa substrate 3001, a light-emitting element 3005 over the substrate 3001,a first sealing region 3007 provided around the light-emitting element3005, and a second sealing region 3009 provided around the first sealingregion 3007.

Light is emitted from the light-emitting element 3005 through one orboth of the substrate 3001 and a substrate 3003. In FIGS. 13A and 13B, astructure in which light is emitted from the light-emitting element 3005to the lower side (the substrate 3001 side) is illustrated.

As illustrated in FIGS. 13A and 13B, the light-emitting device 3000 hasa double sealing structure in which the light-emitting element 3005 issurrounded by the first sealing region 3007 and the second sealingregion 3009. With the double sealing structure, entry of impurities(e.g., water, oxygen, and the like) from the outside into thelight-emitting element 3005 can be favorably suppressed. Note that it isnot necessary to provide both the first sealing region 3007 and thesecond sealing region 3009. For example, only the first sealing region3007 may be provided.

Note that in FIG. 13B, the first sealing region 3007 and the secondsealing region 3009 are each provided in contact with the substrate 3001and the substrate 3003. However, without limitation to such a structure,for example, one or both of the first sealing region 3007 and the secondsealing region 3009 may be provided in contact with an insulating filmor a conductive film provided on the substrate 3001. Alternatively, oneor both of the first sealing region 3007 and the second sealing region3009 may be provided in contact with an insulating film or a conductivefilm provided on the substrate 3003.

The substrate 3001 and the substrate 3003 can have structures similar tothose of the substrate 200 and the substrate 220 described in the aboveembodiment, respectively. The light-emitting element 3005 can have astructure similar to that of any of the light-emitting elementsdescribed in the above embodiments.

For the first sealing region 3007, a material containing glass (e.g., aglass frit, a glass ribbon, and the like) can be used. For the secondsealing region 3009, a material containing a resin can be used. With theuse of the material containing glass for the first sealing region 3007,productivity and a sealing property can be improved. Moreover, with theuse of the material containing a resin for the second sealing region3009, impact resistance and heat resistance can be improved. However,the materials used for the first sealing region 3007 and the secondsealing region 3009 are not limited thereto, and the first sealingregion 3007 may be formed using the material containing a resin and thesecond sealing region 3009 may be formed using the material containingglass.

The glass frit may contain, for example, magnesium oxide, calcium oxide,strontium oxide, barium oxide, cesium oxide, sodium oxide, potassiumoxide, boron oxide, vanadium oxide, zinc oxide, tellurium oxide,aluminum oxide, silicon dioxide, lead oxide, tin oxide, phosphorusoxide, ruthenium oxide, rhodium oxide, iron oxide, copper oxide,manganese dioxide, molybdenum oxide, niobium oxide, titanium oxide,tungsten oxide, bismuth oxide, zirconium oxide, lithium oxide, antimonyoxide, lead borate glass, tin phosphate glass, vanadate glass, orborosilicate glass. The glass frit preferably contains at least one kindof transition metal to absorb infrared light.

As the above glass frits, for example, a frit paste is applied to asubstrate and is subjected to heat treatment, laser light irradiation,or the like. The frit paste contains the glass frit and a resin (alsoreferred to as a binder) diluted by an organic solvent. Note that anabsorber which absorbs light having the wavelength of laser light may beadded to the glass frit. For example, an Nd:YAG laser or a semiconductorlaser is preferably used as the laser. The shape of laser light may becircular or quadrangular.

As the above material containing a resin, for example, polyester,polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate,or an acrylic resin, polyurethane, or an epoxy resin can be used.Alternatively, a material that includes a resin having a siloxane bondsuch as silicone can be used.

Note that in the case where the material containing glass is used forone or both of the first sealing region 3007 and the second sealingregion 3009, the material containing glass preferably has a thermalexpansion coefficient close to that of the substrate 3001. With theabove structure, generation of a crack in the material containing glassor the substrate 3001 due to thermal stress can be suppressed.

For example, the following advantageous effect can be obtained in thecase where the material containing glass is used for the first sealingregion 3007 and the material containing a resin is used for the secondsealing region 3009.

The second sealing region 3009 is provided closer to an outer portion ofthe light-emitting device 3000 than the first sealing region 3007 is. Inthe light-emitting device 3000, distortion due to external force or thelike increases toward the outer portion. Thus, the outer portion of thelight-emitting device 3000 where a larger amount of distortion isgenerated, that is, the second sealing region 3009 is sealed using thematerial containing a resin and the first sealing region 3007 providedon an inner side of the second sealing region 3009 is sealed using thematerial containing glass, whereby the light-emitting device 3000 isless likely to be damaged even when distortion due to external force orthe like is generated.

Furthermore, as illustrated in FIG. 13B, a first region 3011 correspondsto the region surrounded by the substrate 3001, the substrate 3003, thefirst sealing region 3007, and the second sealing region 3009. A secondregion 3013 corresponds to the region surrounded by the substrate 3001,the substrate 3003, the light-emitting element 3005, and the firstsealing region 3007.

The first region 3011 and the second region 3013 are preferably filledwith, for example, an inert gas such as a rare gas or a nitrogen gas.Alternatively, the first region 3011 and the second region 3013 arepreferably filled with a resin such as an acrylic resin or an epoxyresin. Note that for the first region 3011 and the second region 3013, areduced pressure state is preferred to an atmospheric pressure state.

FIG. 13C illustrates a modification example of the structure in FIG.13B. FIG. 13C is a cross-sectional view illustrating the modificationexample of the light-emitting device 3000.

FIG. 13C illustrates a structure in which a desiccant 3018 is providedin a recessed portion provided in part of the substrate 3003. The othercomponents are the same as those of the structure illustrated in FIG.13B.

As the desiccant 3018, a substance which adsorbs moisture and the likeby chemical adsorption or a substance which adsorbs moisture and thelike by physical adsorption can be used. Examples of the substance thatcan be used as the desiccant 3018 include alkali metal oxides, alkalineearth metal oxides (e.g., calcium oxide, barium oxide, and the like),sulfate, metal halides, perchlorate, zeolite, silica gel, and the like.

<Lighting Device>

FIG. 14 illustrates an example in which the light-emitting element isused for an indoor lighting device 8501. Since the light-emittingelement can have a larger area, a lighting device having a large areacan also be formed. In addition, a lighting device 8502 in which alight-emitting region has a curved surface can also be formed with theuse of a housing with a curved surface. The light-emitting elementdescribed in this embodiment is in the form of a thin film, which allowsthe housing to be designed more freely. Therefore, the lighting devicecan be elaborately designed in a variety of ways. Furthermore, a wall ofthe room may be provided with a large-sized lighting device 8503. Touchsensors may be provided in the lighting devices 8501, 8502, and 8503 tocontrol the power on/off of the lighting devices.

Moreover, when the light-emitting element is used on the surface side ofa table, a lighting device 8504 which functions as a table can beobtained. When the light-emitting element is used as part of otherfurniture, a lighting device which functions as the furniture can beobtained.

As described above, display modules, light-emitting devices, electronicdevices, and lighting devices can be obtained by application of thelight-emitting element of one embodiment of the present invention. Notethat the light-emitting element can be used for electronic devices in avariety of fields without being limited to the lighting devices and theelectronic devices described in this embodiment.

The structure described in this embodiment can be combined with any ofthe structures described in the other embodiments as appropriate.

Example 1

In this example, examples of fabrication methods of the light-emittingelement of one embodiment of the present invention and a comparativelight-emitting element will be described. The structure of each of thelight-emitting elements fabricated in this example is the same as thatillustrated in FIG. 1. Table 1 shows the details of the elementstructures. In addition, structures and abbreviations of compounds usedhere are given below.

TABLE 1 Reference Thickness Weight Layer numeral (nm) Material ratioLight-emitting Electrode 102 200 Al — element 1 Electron-injection 119 1LiF — layer Electron-transport 118 (2) 10 BPhen — layer 118 (1) 204,6mCzP2Pm — Light-emitting layer 130 40 4,6mCzP2Pm:Ir(dmpimpt-Me)₃1:0.1 Hole-transport layer 112 20 mCzFLP — Hole-injection layer 111 60DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — Comparative Electrode 102200 Al — light-emitting Electron-injection 119 1 LiF — element 1 layerElectron-transport 118 (2) 15 BPhen — layer 118 (1) 10mDBTBIm-II:Ir(dmpimpt-Me)₃  1:0.08 Light-emitting layer 130 30mCP:Ir(dmpimpt-Me)₃  1:0.08 Hole-transport layer 112 20 mCP —Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5 Electrode 101 110 ITSO —<Fabrication of Light-Emitting Elements>

Methods for fabricating the light-emitting elements fabricated in thisexample will be described below.

<<Fabrication of Light-Emitting Element 1>>

As the electrode 101, an ITSO film was formed to a thickness of 70 nmover a glass substrate. The electrode area of the electrode 101 was setto 4 mm² (2 mm×2 mm).

As the hole-injection layer 111, DBT3P-II and molybdenum oxide (MoO₃)were deposited to a thickness of 60 nm over the electrode 101 byco-evaporation at a weight ratio of 1:0.5 (DBT3P-II:MoO₃).

As the hole-transport layer 112,9-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-9H-carbazole (abbreviation:mCzFLP) was deposited over the hole-injection layer 111 by evaporationto a thickness of 20 nm.

As the light-emitting layer 130, 4,6mCzP2Pm and Ir(dmpimpt-Me)₃ weredeposited to a thickness of 40 nm over the hole-transport layer 112 byco-evaporation at a weight ratio of 1:0.1 (4,6mCzP2Pm:Ir(dmpimpt-Me)₃).In the light-emitting layer 130, Ir(dmpimpt-Me)₃ was a phosphorescentcompound corresponding to the first organic compound, and 4,6mCzP2Pmcorresponded to the second organic compound.

As the electron-transport layer 118, 4,6mCzP2Pm and Bphen weresequentially deposited by evaporation to thicknesses of 20 nm and 10 nm,respectively, over the light-emitting layer 130. Then, as theelectron-injection layer 119, LiF was deposited over theelectron-transport layer 118 by evaporation to a thickness of 1 nm.

As the electrode 102, aluminum (Al) was formed over theelectron-injection layer 119 to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, the electrodesand the EL layer were sealed by fixing a glass substrate for sealing tothe glass substrate on which the organic materials were deposited usinga sealant for an organic EL device. Specifically, after the sealant wasapplied so as to surround the organic materials deposited on the glasssubstrate and the glass substrate was bonded to the glass substrate forsealing, irradiation with ultraviolet light having a wavelength of 365nm at 6 J/cm² and heat treatment at 80° C. for one hour were performed.Through the above process, the light-emitting element 1 was fabricated.

<<Fabrication of Comparative Light-Emitting Element 1>>

As the electrode 101, an ITSO film was formed to a thickness of 110 nmover a glass substrate. The electrode area of the electrode 101 was setto 4 mm² (2 mm×2 mm).

As the hole-injection layer 111, DBT3P-II and molybdenum oxide (MoO₃)were deposited to a thickness of 60 nm over the electrode 101 byco-evaporation at a weight ratio of 1:0.5 (DBT3P-II:MoO₃).

As the hole-transport layer 112, mCP was deposited over thehole-injection layer 111 by evaporation to a thickness of 20 nm.

As the light-emitting layer 130, mCP and Ir(dmpimpt-Me)₃ were depositedto a thickness of 30 nm over the hole-transport layer 112 byco-evaporation at a weight ratio of 1:0.08 (mCP:Ir(dmpimpt-Me)₃). In thelight-emitting layer 130, Ir(dmpimpt-Me)₃ was a phosphorescent compoundcorresponding to a guest material and mCP corresponded to a hostmaterial.

As the electron-transport layer 118, mDBTBIm-II and Ir(dmpimpt-Me)₃ weredeposited to a thickness of 10 nm over the light-emitting layer 130 byco-evaporation at a weight ratio of 1:0.08 (mDBTBIm-II:Ir(dmpimpt-Me)₃),and then BPhen was deposited by evaporation to a thickness of 15 nm.Then, as the electron-injection layer 119, LiF was deposited over theelectron-transport layer 118 by evaporation to a thickness of 1 nm.

As the electrode 102, aluminum (Al) was formed over theelectron-injection layer 119 to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, the electrodesand the EL layer were sealed by fixing a glass substrate for sealing tothe glass substrate on which the organic materials were deposited usinga sealant for an organic EL device. For the detailed method, thedescription of the light-emitting element 1 can be referred to. Throughthe above process, the comparative light-emitting element 1 wasfabricated.

<Characteristics of Light-Emitting Elements>

Next, the characteristics of the fabricated light-emitting element 1 andcomparative light-emitting element 1 were measured. Luminances and CIEchromaticities were measured with a luminance colorimeter (BM-5Aproduced by TOPCON TECHNOHOUSE CORPORATION), and electroluminescencespectra were measured with a multi-channel spectrometer (PMA-11 producedby Hamamatsu Photonics K.K.).

FIG. 15 shows the luminance-current density characteristics of thelight-emitting element 1 and the comparative light-emitting element 1.FIG. 16 shows the luminance-voltage characteristics thereof. FIG. 17shows the current efficiency-luminance characteristics thereof. FIG. 18shows the power efficiency-luminance characteristics thereof. FIG. 19shows the external quantum efficiency-luminance characteristics thereof.FIG. 20 shows the electroluminescence spectra obtained when a current ata current density of 2.5 mA/cm² was supplied to the light-emittingelement 1 and the comparative light-emitting element 1. The measurementsof the light-emitting elements were performed at room temperature (in anatmosphere kept at 23° C.).

Table 2 shows the element characteristics of the light-emitting element1 and the comparative light-emitting element 1 at around 500 cd/m².

TABLE 2 External Current CIE Current Power quantum Voltage densitychromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 3.30 1.01 (0.528, 0.468) 44043.1 41.0 15.4 element 1 Comparative light- 6.00 2.05 (0.167, 0.239) 41019.8 10.4 12.1 emitting element 1

As shown in FIG. 20, the electroluminescence spectrum of thelight-emitting element 1 has a peak at a wavelength of 545 nm and has alarge full width at half maximum of 105 nm. This indicates that thelight-emitting element 1 emits yellow light. Meanwhile, theelectroluminescence spectrum of the comparative light-emitting element 1has a peak at a wavelength of 460 nm and has a small full width at halfmaximum of 52 nm. This indicates that the comparative light-emittingelement 1 emits blue light. The light emission of the comparativelight-emitting element 1 originated from a phosphorescent compoundIr(dmpimpt-Me)₃. Meanwhile, no light emission originating from aphosphorescent compound Ir(dmpimpt-Me)₃ was observed from thelight-emitting element 1.

As shown in FIG. 15 to FIG. 19 and Table 2, the driving voltage of thelight-emitting element 1 is lower than that of the comparativelight-emitting element 1. Furthermore, the light-emitting element 1 hashigher luminous efficiency (current efficiency, power efficiency, andexternal quantum efficiency) than the comparative light-emitting element1. Accordingly, the light-emitting element of one embodiment of thepresent invention that contains the first organic compound(Ir(dmpimpt-Me)₃) and the second organic compound (4,6mCzP2Pm) is alight-emitting element with high luminous efficiency that drives at lowdriving voltage at low power consumption.

<CV Measurement Results>

The electrochemical characteristics (oxidation reaction characteristicsand reduction reaction characteristics) of the above compounds weremeasured by cyclic voltammetry (CV) measurement. Note that for themeasurement, an electrochemical analyzer (ALS model 600A or 600C,produced by BAS Inc.) was used, and the measurement was performed on asolution obtained by dissolving each compound in N,N-dimethylformamide(abbreviation: DMF). In the measurement, the potential of a workingelectrode with respect to the reference electrode was changed within anappropriate range, so that the oxidation peak potential and thereduction peak potential were obtained. In addition, the HOMO and LUMOlevels of each compound were calculated from the estimated redoxpotential of the reference electrode (−4.94 eV) and the obtained peakpotentials.

According to the CV measurement results, the oxidation potential and thereduction potential of 4,6mCzP2Pm were 0.95 V and −2.06 V, respectively.The HOMO level and the LUMO level of 4,6mCzP2Pm calculated from the CVmeasurement results were −5.89 eV and −2.88 eV, respectively. Thus, itis found that 4,6mCzP2Pm has a low LUMO level. The oxidation potentialand the reduction potential of Ir(dmpimpt-Me)₃ were 0.24 V and −2.67 V,respectively. The HOMO level and the LUMO level of Ir(dmpimpt-Me)₃calculated from the CV measurement results were −5.18 eV and −2.27 eV,respectively. Thus, it is found that Ir(dmpimpt-Me)₃ has a high HOMOlevel. The oxidation potential of mCP was 0.97 V. The HOMO level of mCPcalculated from the CV measurement results was −5.91 eV. Note that theLUMO level of mCP is probably high because the reduction potential ofmCP was low and no clear reduction peak was observed.

As described above, the LUMO level and the HOMO level of 4,6mCzP2Pm,which is the second organic compound, are lower than those ofIr(dmpimpt-Me)₃, which is the first organic compound. Thus, in the casewhere the compounds are used in a light-emitting layer as in thelight-emitting element 1, electrons and holes, which serve as carriers,are efficiently injected from a pair of electrodes into 4,6mCzP2Pm (thesecond organic compound) and Ir(dmpimpt-Me)₃ (the first organiccompound), respectively, so that 4,6mCzP2Pm (the second organiccompound) and Ir(dmpimpt-Me)₃ (the first organic compound) can form anexciplex.

The exciplex formed by 4,6mCzP2Pm and Ir(dmpimpt-Me)₃ has the LUMO levelin 4,6mCzP2Pm and the HOMO level in Ir(dmpimpt-Me)₃. The energydifference between the LUMO level of 4,6mCzP2Pm and the HOMO level ofIr(dmpimpt-Me)₃ is 2.30 eV. This value is substantially equal to lightemission energy (2.28 eV) calculated from the peak wavelength of theelectroluminescence spectrum of the light-emitting element 1 in FIG. 20.This result implies that the electroluminescence spectrum of thelight-emitting element 1 corresponds to light emission due to theexciplex formed by 4,6mCzP2Pm and Ir(dmpimpt-Me)₃. In the exciplex, thedifference between the S1 level and the T1 level is small; thus, thelight emission energy can be regarded as energy of each of the S1 leveland the T1 level (2.28 eV).

Meanwhile, the CV measurement results indicate that the HOMO level ofIr(dmpimpt-Me)₃ is higher than that of mCP and the LUMO level of mCP isprobably higher than that of Ir(dmpimpt-Me)₃. Thus, when the compoundsare used for a light-emitting layer as in the comparative light-emittingelement 1, both electrons and holes serving as carriers injected from apair of electrodes are injected into Ir(dmpimpt-Me)₃, resulting in lightemission from Ir(dmpimpt-Me)₃.

In the light-emitting element 1, an exciplex formed by 4,6mCzP2Pm andIr(dmpimpt-Me)₃ emits light; thus, the exciplex can be formed withenergy corresponding to the difference between the LUMO level of4,6mCzP2Pm and the HOMO level of Ir(dmpimpt-Me)₃ (i.e., 2.30 eV). Incontrast, in the comparative light-emitting element 1, Ir(dmpimpt-Me)₃is excited to emit light; thus, at least energy corresponding to thedifference between the LUMO level and the HOMO level of Ir(dmpimpt-Me)₃(i.e., 2.91 eV) is needed for excitation. Therefore, the light-emittingelement 1 can emit light at lower driving voltage than the comparativelight-emitting element 1.

Furthermore, the excitation energy level of the exciplex formed by4,6mCzP2Pm and Ir(dmpimpt-Me)₃ is smaller than the energy differencebetween the LUMO level and the HOMO level of 4,6mCzP2Pm (i.e., 3.01 eV).Thus, when the exciplex is formed, a light-emitting element with lowdriving voltage can be obtained.

<Measurement of T1 Level>

Next, to obtain the T1 levels of the compounds each used in thelight-emitting layer 130, a thin film of 4,6mCzP2Pm and a thin film ofmCP were each formed over a quartz substrate by a vacuum evaporationmethod, and the emission spectra of the thin films were measured at alow temperature (10 K).

The measurement was performed with a PL microscope and LabRAM HR-PL,produced by HORIBA, Ltd., at a measurement temperature of 10 K using aHe—Cd laser having a wavelength of 325 nm as excitation light and a CCDdetector as a detector.

In the measurement method of the emission spectra, in addition to thenormal measurement of emission spectra, the measurement of time-resolvedemission spectra in which light emission with a long lifetime is focusedon was performed. Since the measurement temperature in this measurementmethod of emission spectra was set at a low temperature (10 K,phosphorescence was partly observed in the normal measurement ofemission spectra, in addition to fluorescence, which is the mainemission component. Furthermore, in the measurement of time-resolvedemission spectra in which light emission with a long lifetime is focusedon, phosphorescence was mainly observed. FIG. 21 and FIG. 22 show thetime-resolved emission spectra of 4,6mCzP2Pm and mCP, respectively,measured at a low temperature.

As shown in the measurement results of the emission spectra, theemission spectrum of 4,6mCzP2Pm has a peak (including a shoulder) of thephosphorescent component on the shortest wavelength side at 459 nm, andthe emission spectrum of mCP has a peak (including a shoulder) of thephosphorescent component on the shortest wavelength side at 421 nm.

Thus, from the peak wavelengths, the T1 level of 4,6mCzP2Pm wascalculated to be 2.70 eV and the T1 level of mCP was calculated to be2.95 eV.

<Absorption Spectrum and Emission Spectrum of Compound>

Next, FIG. 23 shows the measurement results of the absorption andemission spectra of Ir(dmpimpt-Me)₃.

For the measurement of the absorption and emission spectra, adichloromethane solution in which Ir(dmpimpt-Me)₃ was dissolved wasprepared, and a quartz cell was used. The absorption spectrum wasmeasured using an ultraviolet-visible spectrophotometer (V-550, producedby JASCO Corporation). The absorption spectra of the quartz cell and thesolvent were subtracted from the measured spectrum of the solution. Notethat the emission spectrum of the solution was measured with a PL-ELmeasurement apparatus (produced by Hamamatsu Photonics K.K.). Themeasurement was performed at room temperature (in an atmosphere kept at23° C.).

As shown in FIG. 23, an absorption edge on the lowest energy side (thelongest wavelength side) of the absorption spectrum of Ir(dmpimpt-Me)₃is at around 450 nm. The absorption edge was obtained from data of theabsorption spectrum, and transition energy was estimated on theassumption of direct transition, whereby transition energy ofIr(dmpimpt-Me)₃ was calculated to be 2.71 eV. Since Ir(dmpimpt-Me)₃ is aphosphorescent compound, the absorption band on the lowest energy sideis based on the transition to the triplet excited state. Therefore, theT1 level of Ir(dmpimpt-Me)₃ is calculated to be 2.71 eV.

From the above measurement results, it is found that the T1 level of4,6mCzP2Pm (2.70 eV) is equivalent to the T1 level of Ir(dmpimpt-Me)₃(2.71 eV), and the T1 level of Ir(dmpimpt-Me)₃ (2.71 eV) is higher thanthe T1 level of the exciplex (2.28 eV) formed by 4,6mCzP2Pm andIr(dmpimpt-Me)₃ and is greater than the difference between the LUMOlevel of 4,6mCzP2Pm and the HOMO level of Ir(dmpimpt-Me)₃ (2.30 eV).Thus, the exciplex formed by 4,6mCzP2Pm and Ir(dmpimpt-Me)₃ can emitlight efficiently.

<Luminescence Quantum Yield of Compound>

Next, the luminescence quantum yield of Ir(dmpimpt-Me)₃ was measured.The luminescence quantum yield was measured using a toluene solution(1×10⁻⁵ M) in which Ir(dmpimpt-Me)₃ was dissolved with an absolutequantum yield measurement system C9920-02, produced by HamamatsuPhotonics K.K. The excitation wavelength was in a range of 350 nm to 550nm.

According to the measurement results, the luminescence quantum yield ofIr(dmpimpt-Me)₃ was 7%. This indicates that Ir(dmpimpt-Me)₃ is aphosphorescent material with a low luminescence quantum yield.

The light-emitting element 1 emits light originating from the exciplexformed by 4,6mCzP2Pm and Ir(dmpimpt-Me)₃, and has higher luminousefficiency than the comparative light-emitting element 1 which emitslight originating from Ir(dmpimpt-Me)₃. Since the generation probabilityof singlet excitons which are generated by recombination of carriers(holes and electrons) injected from the pair of electrodes is at most25%, the external quantum efficiency in the case where the lightextraction efficiency to the outside is 30% is at most 7.5%. Thelight-emitting element 1 has external quantum efficiency of more than7.5%. This is because the light-emitting element 1 emits, in addition tolight originating from singlet excitons generated by recombination ofcarriers (holes and electrons) injected from the pair of electrodes,light originating from triplet excitons or light originating fromsinglet excitons generated from triplet excitons by reverse intersystemcrossing in the exciplex. That is, even when a compound with a lowluminescence quantum yield is used, one embodiment of the presentinvention can provide a light-emitting element with high luminousefficiency.

With one embodiment of the present invention, a light-emitting elementwith high luminous efficiency can be provided. With one embodiment ofthe present invention, a light-emitting element emitting light in abroad emission spectrum can be provided. With one embodiment of thepresent invention, a light-emitting element with low driving voltage andlow power consumption can be provided.

Example 2

In this example, an example of a fabrication method of thelight-emitting element of one embodiment of the present invention willbe described. The structure of the light-emitting element fabricated inthis example is the same as that illustrated in FIG. 1. Table 3 showsthe details of the element structure. In addition, a structure and anabbreviation of a compound used here are given below. Note that Example1 is referred to for structures and abbreviations of other compounds.

TABLE 3 Reference Thickness Weight Layer numeral (nm) Material ratioLight-emitting Electrode 102 200 Al — element 2 Electron-injection 119 1LiF — layer Electron-transport 118 (2) 10 BPhen — layer 118 (1) 204,6mCzP2Pm — Light-emitting layer 130 40 4,6mCzP2Pm:Ir(ppz)₃ 1:0.2Hole-transport layer 112 20 mCzFLP — Hole-injection layer 111 60DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO —<Fabrication of Light-Emitting Element 2>

A method for fabricating a light-emitting element 2 fabricated in thisexample will be described below. The light-emitting element 2 wasfabricated through the same steps as those for the above-describedlight-emitting element 1 except for the step of forming thelight-emitting layer 130.

As the light-emitting layer 130 of the light-emitting element 2,4,6mCzP2Pm and tris[2-(1H-pyrazol-1-yl-κN²)phenyl-κC]iridium(III)(abbreviation: Ir(ppz)₃) were deposited to a thickness of 40 nm byco-evaporation at a weight ratio of 1:0.2 (4,6mCzP2Pm:Ir(ppz)₃). In thelight-emitting layer 130, Ir(ppz)₃ corresponded to the first organiccompound, and 4,6mCzP2Pm corresponded to the second organic compound.

<Characteristics of Light-Emitting Element>

Next, the characteristics of the fabricated light-emitting element 2were measured. Note that the measurement method was similar to that usedin Example 1.

FIG. 24 shows the luminance-current density characteristics of thelight-emitting element 2. FIG. 25 shows the luminance-voltagecharacteristics thereof. FIG. 26 shows the current efficiency-luminancecharacteristics thereof. FIG. 27 shows the power efficiency-luminancecharacteristics thereof. FIG. 28 shows the external quantumefficiency-luminance characteristics thereof. FIG. 29 shows theelectroluminescence spectrum obtained when a current at a currentdensity of 2.5 mA/cm² was supplied to the light-emitting element 2. Themeasurements of the light-emitting element were performed at roomtemperature (in an atmosphere kept at 23° C.).

Table 4 shows the element characteristics of the light-emitting element2 at around 1000 cd/m².

TABLE 4 External Current CIE Current Power quantum Voltage densitychromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 3.60 1.42 (0.348, 0.585) 93065.6 57.2 19.4 element 2

As shown in FIG. 29, the electroluminescence spectrum of thelight-emitting element 2 has a peak at a wavelength of 537 nm and a fullwidth at half maximum of 84 nm. This indicates that the light-emittingelement 2 emits yellow light. Note that Ir(ppz)₃ used in thelight-emitting element 2 is known as a compound which emits blue lightat low temperatures; however, light originating from Ir(ppz)₃ was notobserved here.

As shown in FIG. 24 to FIG. 28 and Table 4, the driving voltage of thelight-emitting element 2 is low. Furthermore, the light-emitting element2 has high luminous efficiency (current efficiency, power efficiency,and external quantum efficiency). Accordingly, the light-emittingelement of one embodiment of the present invention that contains thefirst organic compound (Ir(ppz)₃) and the second organic compound(4,6mCzP2Pm) is a light-emitting element with high luminous efficiencythat drives at low driving voltage at low power consumption.

<CV Measurement Results>

The electrochemical characteristics (oxidation reaction characteristicsand reduction reaction characteristics) of the above compounds weremeasured by cyclic voltammetry (CV) measurement. Note that themeasurement method was similar to that used in Example 1. Example 1 isreferred to for the measurement results of 4,6mCzP2Pm.

According to the CV measurement results, the oxidation potential and thereduction potential of Ir(ppz)₃ were 0.45 V and −3.17 V, respectively.The HOMO level and the LUMO level of Ir(ppz)₃ calculated from the CVmeasurement results were −5.39 eV and −1.77 eV, respectively. Thus, itis found that Ir(ppz)₃ has a high HOMO level.

As described above, the LUMO level and the HOMO level of 4,6mCzP2Pm,which is the second organic compound, are lower than those of Ir(ppz)₃,which is the first organic compound. Thus, in the case where thecompounds are used in a light-emitting layer as in the light-emittingelement 2, electrons and holes, which serve as carriers, are efficientlyinjected from a pair of electrodes into 4,6mCzP2Pm (the second organiccompound) and Ir(ppz)₃ (the first organic compound), respectively, sothat 4,6mCzP2Pm (the second organic compound) and Ir(ppz)₃ (the firstorganic compound) can form an exciplex.

The difference between the LUMO level of 4,6mCzP2Pm and the HOMO levelof Ir(ppz)₃ is 2.51 eV. This value is substantially equal to lightemission energy (2.31 eV) calculated from the peak wavelength of theelectroluminescence spectrum of the light-emitting element 2 in FIG. 29.This result implies that the electroluminescence spectrum of thelight-emitting element 2 corresponds to light emission due to theexciplex formed by 4,6mCzP2Pm and Ir(ppz)₃. In the exciplex, thedifference between the S1 level and the T1 level is small; thus, thelight emission energy can be regarded as energy of each of the S1 leveland the T1 level (2.31 eV).

Furthermore, the excitation energy level of the exciplex formed by4,6mCzP2Pm and Ir(ppz)₃ is smaller than the energy difference betweenthe LUMO level and the HOMO level of 4,6mCzP2Pm (i.e., 3.01 eV). Thus,when the exciplex is formed, a light-emitting element with low drivingvoltage can be obtained.

<Absorption Spectrum of Compound>

Next, FIG. 30 shows the measurement results of the absorption spectrumof Ir(ppz)₃.

For the measurement of the absorption spectrum, a dichloromethanesolution in which Ir(ppz)₃ was dissolved was prepared, and a quartz cellwas used. The absorption spectrum was measured using anultraviolet-visible spectrophotometer (V-550, produced by JASCOCorporation). The absorption spectra of the quartz cell and the solventwere subtracted from the measured spectrum of the solution. Themeasurement was performed at room temperature (in an atmosphere kept at23° C.).

As shown in FIG. 30, an absorption edge on the lowest energy side (thelongest wavelength side) of the absorption spectrum of Ir(ppz)₃ is ataround 370 nm. The absorption edge was obtained from data of theabsorption spectrum, and transition energy was estimated on theassumption of direct transition, whereby transition energy of Ir(ppz)₃was calculated to be 3.27 eV. Since Ir(ppz)₃ is a phosphorescentcompound, the absorption band on the lowest energy side is based on thetransition to the triplet excited state. Therefore, from the absorptionedge, the T1 level of Ir(ppz)₃ is calculated to be 3.27 eV.

From the above measurement results, it is found that the T1 level of4,6mCzP2Pm (2.70 eV) is lower than the T1 level of Ir(ppz)₃ (3.27 eV),higher than the T1 level of the exciplex (2.31 eV) formed by 4,6mCzP2Pmand Ir(ppz)₃, and is greater than the difference between the LUMO levelof 4,6mCzP2Pm and the HOMO level of Ir(ppz)₃ (2.51 eV). Thus, theexciplex formed by 4,6mCzP2Pm and Ir(ppz)₃ can emit light efficiently.

In addition, when the measurement of emission spectrum of Ir(ppz)₃ wasperformed at room temperature, light emission from Ir(ppz)₃ was notobserved. Non-Patent Document 1 discloses that the luminescence quantumyield of Ir(ppz)₃ is lower than 1% at room temperature. This indicatesthat Ir(ppz)₃ is a material that does not emit light at roomtemperature.

The light-emitting element 2 which emits light originating from theexciplex formed by 4,6mCzP2Pm and Ir(ppz)₃ has high external quantumefficiency, which is higher than 20%. This is because the light-emittingelement 2 emits, in addition to light originating from singlet excitonsgenerated by recombination of carriers (holes and electrons) injectedfrom the pair of electrodes, light originating from triplet excitons orlight originating from singlet excitons generated from triplet excitonsby reverse intersystem crossing in the exciplex. That is, even when acompound with a low luminescence quantum yield, which is lower than 1%,is used, one embodiment of the present invention can provide alight-emitting element with high luminous efficiency.

With one embodiment of the present invention, a light-emitting elementwith high luminous efficiency can be provided. With one embodiment ofthe present invention, a light-emitting element with low driving voltageand low power consumption can be provided.

Example 3

In this example, examples of fabrication methods of the light-emittingelements of one embodiment of the present invention will be described.The structure of each of the light-emitting elements fabricated in thisexample is the same as that illustrated in FIG. 1. Table 5 shows thedetails of the element structures. In addition, structures andabbreviations of compounds used here are given below. Note that Examples1 and 2 are referred to for structures and abbreviations of othercompounds.

TABLE 5 Reference Thickness Weight Layer numeral (nm) Material ratioLight-emitting Electrode 102 200 Al — element 3 Electron-injection 119 1LiF — layer Electron-transport 118 (2) 10 BPhen — layer 118 (1) 204,6mCzP2Pm — Light-emitting layer 130 40 4,6mCzP2Pm:Ir(ppz)₃ 1:0.1Hole-transport layer 112 20 mCzFLP — Hole-injection layer 111 60DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — Light-emitting Electrode 102200 Al — element 4 Electron-injection 119 1 LiF — layerElectron-transport 118 (2) 10 BPhen — layer 118 (1) 20 4,6mCzBP2Pm —Light-emitting layer 130 40 4,6mCzBP2Pm:Ir(ppz)₃ 1:0.1 Hole-transportlayer 112 20 mCzFLP — Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5Electrode 101 70 ITSO — Light-emitting Electrode 102 200 Al — element 5Electron-injection 119 1 LiF — layer Electron-transport 118 (2) 10 BPhen— layer 118 (1) 20 5Me-4,6mCzP2Pm — Light-emitting layer 130 405Me-4,6mCzP2Pm:Ir(ppz)₃ 1:0.1 Hole-transport layer 112 20 mCzFLP —Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO —Light-emitting Electrode 102 200 Al — element 6 Electron-injection 119 1LiF — layer Electron-transport 118 (2) 10 BPhen — layer 118 (1) 204,4′mCzP2BPy — Light-emitting layer 130 40 4,4′mCzP2BPy:Ir(ppz)₃ 1:0.1Hole-transport layer 112 20 mCzFLP — Hole-injection layer 111 60DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO —<Fabrication of Light-Emitting Elements 3 to 6>

Methods for fabricating the light-emitting elements 3 to 6 fabricated inthis example will be described below. The light-emitting elements 3 to 6were fabricated through the same steps as those for the above-describedlight-emitting element 1 except for the steps of forming thelight-emitting layer 130 and the electron-transport layer 118.

As the light-emitting layer 130 of the light-emitting element 3,4,6mCzP2Pm and Ir(ppz)₃ were deposited to a thickness of 40 nm byco-evaporation at a weight ratio of 1:0.1 (4,6mCzP2Pm:Ir(ppz)₃). In thelight-emitting layer 130, Ir(ppz)₃ corresponded to the first organiccompound, and 4,6mCzP2Pm corresponded to the second organic compound.Then, as the electron-transport layer 118, 4,6mCzP2Pm and Bphen weresequentially deposited by evaporation to thicknesses of 20 nm and 10 nm,respectively, over the light-emitting layer 130.

As the light-emitting layer 130 of the light-emitting element 4,9,9′-[pyrimidine-4,6-diylbis(biphenyl-3,3′-diyl)]bis(9H-carbazole)(abbreviation: 4,6mCzBP2Pm) and Ir(ppz)₃ were deposited to a thicknessof 40 nm by co-evaporation at a weight ratio of 1:0.1(4,6mCzBP2Pm:Ir(ppz)₃). In the light-emitting layer 130, Ir(ppz)₃corresponded to the first organic compound, and 4,6mCzBP2Pm correspondedto the second organic compound. Then, as the electron-transport layer118, 4,6mCzBP2Pm and Bphen were sequentially deposited by evaporation tothicknesses of 20 nm and 10 nm, respectively, over the light-emittinglayer 130.

As the light-emitting layer 130 of the light-emitting element 5,5-methyl-4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation:5Me-4,6mCzP2Pm) and Ir(ppz)₃ were deposited to a thickness of 40 nm byco-evaporation at a weight ratio of 1:0.1 (5Me-4,6mCzP2Pm:Ir(ppz)₃). Inthe light-emitting layer 130, Ir(ppz)₃ corresponded to the first organiccompound, and 5Me-4,6mCzP2Pm corresponded to the second organiccompound. Then, as the electron-transport layer 118, 5Me-4,6mCzP2Pm andBphen were sequentially deposited by evaporation to thicknesses of 20 nmand 10 nm, respectively, over the light-emitting layer 130.

As the light-emitting layer 130 of the light-emitting element 6,4,4′-bis[3-(9H-carbazol-9-yl)phenyl]-2,2′-bipyridine (abbreviation:4,4′mCzP2BPy) and Ir(ppz)₃ were deposited to a thickness of 40 nm byco-evaporation at a weight ratio of 1:0.1 (4,4′mCzP2BPy:Ir(ppz)₃). Inthe light-emitting layer 130, Ir(ppz)₃ corresponded to the first organiccompound, and 4,4′mCzP2BPy corresponded to the second organic compound.Then, as the electron-transport layer 118, 4,4′mCzP2BPy and Bphen weresequentially deposited by evaporation to thicknesses of 20 nm and 10 nm,respectively, over the light-emitting layer 130.

<Characteristics of Light-Emitting Elements>

Next, the characteristics of the fabricated light-emitting elements 3 to6 were measured. Note that the measurement method was similar to thatused in Example 1.

FIG. 31 shows the luminance-current density characteristics of thelight-emitting elements 3 to 6. FIG. 32 shows the luminance-voltagecharacteristics thereof. FIG. 33 shows the current efficiency-luminancecharacteristics thereof. FIG. 34 shows the power efficiency-luminancecharacteristics thereof. FIG. 35 shows the external quantumefficiency-luminance characteristics thereof. FIG. 36 shows theelectroluminescence spectra obtained when a current at a current densityof 2.5 mA/cm² was supplied to the light-emitting elements 3 to 6. Themeasurements of the light-emitting elements were performed at roomtemperature (in an atmosphere kept at 23° C.).

Table 6 shows the element characteristics of the light-emitting elements3 to 6 at around 1000 cd/m².

TABLE 6 External Current CIE Current Power quantum Voltage densitychromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 3.60 1.57 (0.327, 0.580) 97061.3 53.5 18.6 element 3 Light-emitting 4.40 4.78 (0.310, 0.565) 113023.6 16.8 7.36 element 4 Light-emitting 5.80 10.2 (0.287, 0.531) 106010.4 5.64 3.46 element 5 Light-emitting 6.60 62.7 (0.262, 0.498) 9401.50 0.712 0.524 element 6

As shown in FIG. 36, the electroluminescence spectra of thelight-emitting elements 3, 4, 5, and 6 have peaks at wavelengths of 526nm, 521 nm, 517 nm, and 508 nm, respectively, and have full widths athalf maximum of 95 nm, 96 nm, 100 nm, and 94 nm, respectively. Thisindicates that the light-emitting elements 3 to 6 emit yellow light.Note that Ir(ppz)₃ used in the light-emitting elements 3 to 6 is knownas a compound which emits blue light at low temperatures; however, lightoriginating from Ir(ppz)₃ was not observed here.

As shown in FIG. 31 to FIG. 35 and Table 6, the driving voltage of thelight-emitting element 3 is low. The light-emitting element 3, thelight-emitting element 4, the light-emitting element 5, and thelight-emitting element 6 have low driving voltage in this order. Thelight-emitting element 3 has high luminous efficiency (currentefficiency, power efficiency, and external quantum efficiency). Thelight-emitting element 3, the light-emitting element 4, thelight-emitting element 5, and the light-emitting element 6 have highluminous efficiency in this order.

<CV Measurement Results>

The electrochemical characteristics (oxidation reaction characteristicsand reduction reaction characteristics) of the above compounds weremeasured by cyclic voltammetry (CV) measurement. Note that themeasurement method was similar to that used in Example 1. Examples 1 and2 are referred to for the measurement results of 4,6mCzP2Pm andIr(ppz)₃.

According to the CV measurement results, the oxidation potential and thereduction potential of 4,6mCzBP2Pm were 0.95 V and −2.14 V,respectively. The HOMO level and the LUMO level of 4,6mCzBP2Pmcalculated from the CV measurement results were −5.89 eV and −2.80 eV,respectively. Thus, it is found that 4,6mCzBP2Pm has a low LUMO level.

According to the CV measurement results, the oxidation potential and thereduction potential of 5Me-4,6mCzP2Pm were 0.97 V and −2.22 V,respectively. The HOMO level and the LUMO level of 5Me-4,6mCzP2Pmcalculated from the CV measurement results were −5.91 eV and −2.73 eV,respectively. Thus, it is found that 5Me-4,6mCzP2Pm has a low LUMOlevel.

According to the CV measurement results, the oxidation potential and thereduction potential of 4,4′mCzP2BPy were 1.00 V and −2.29 V,respectively. The HOMO level and the LUMO level of 4,4′mCzP2BPycalculated from the CV measurement results were −5.94 eV and −2.66 eV,respectively. Thus, it is found that 4,4′mCzP2BPy has a low LUMO level.

As described above, the LUMO level of each of 4,6mCzP2Pm, 4,6mCzBP2Pm,5Me-4,6mCzP2Pm, and 4,4′mCzP2BPy, which are the second organiccompounds, is lower than the LUMO level of Ir(ppz)₃, which is the firstorganic compound. Furthermore, the HOMO level of each of 4,6mCzP2Pm,4,6mCzBP2Pm, 5Me-4,6mCzP2Pm, and 4,4′mCzP2BPy, which are the secondorganic compounds, is lower than the HOMO level of Ir(ppz)₃, which isthe first organic compound. Thus, in the case where the compounds areused in a light-emitting layer as in the light-emitting elements 3 to 6,electrons and holes, which serve as carriers, are efficiently injectedfrom a pair of electrodes into the second organic compound (4,6mCzP2Pm,4,6mCzBP2Pm, 5Me-4,6mCzP2Pm, or 4,4′mCzP2BPy) and the first organiccompound (Ir(ppz)₃), respectively, so that the second organic compound(4,6mCzP2Pm, 4,6mCzBP2Pm, 5Me-4,6mCzP2Pm, or 4,4′mCzP2BPy) and the firstorganic compound can form an exciplex.

The exciplex formed by the second organic compound (4,6mCzP2Pm,4,6mCzBP2Pm, 5Me-4,6mCzP2Pm, or 4,4′mCzP2BPy) and the first organiccompound (Ir(ppz)₃) has the LUMO level in the second organic compound(4,6mCzP2Pm, 4,6mCzBP2Pm, 5Me-4,6mCzP2Pm, or 4,4′mCzP2BPy) and the HOMOlevel in the first organic compound (Ir(ppz)₃).

The energy difference between the LUMO level of 4,6mCzBP2Pm and the HOMOlevel of Ir(ppz)₃ is 2.59 eV. This value is substantially equal to lightemission energy (2.38 eV) calculated from the peak wavelength of theelectroluminescence spectrum of the light-emitting element 4 in FIG. 36.This result implies that the electroluminescence spectrum of thelight-emitting element 4 corresponds to light emission due to theexciplex formed by 4,6mCzBP2Pm and Ir(ppz)₃. In the exciplex, thedifference between the S1 level and the T1 level is small; thus, thelight emission energy can be regarded as energy of each of the S1 leveland the T1 level (2.38 eV). Furthermore, the excitation energy level ofthe exciplex formed by 4,6mCzBP2Pm and Ir(ppz)₃ is smaller than theenergy difference between the LUMO level and the HOMO level of4,6mCzBP2Pm (3.09 eV). Thus, when the exciplex is formed, alight-emitting element with low driving voltage can be obtained.

The energy difference between the LUMO level of 5Me-4,6mCzP2Pm and theHOMO level of Ir(ppz)₃ is 2.66 eV. This value is substantially equal tolight emission energy (2.40 eV) calculated from the peak wavelength ofthe electroluminescence spectrum of the light-emitting element 5 in FIG.36. This result implies that the electroluminescence spectrum of thelight-emitting element 5 corresponds to light emission due to theexciplex formed by 5Me-4,6mCzP2Pm and Ir(ppz)₃. In the exciplex, thedifference between the S1 level and the T1 level is small; thus, thelight emission energy can be regarded as energy of each of the S1 leveland the T1 level (2.40 eV). Furthermore, the excitation energy level ofthe exciplex formed by 5Me-4,6mCzP2Pm and Ir(ppz)₃ is smaller than theenergy difference between the LUMO level and the HOMO level of5Me-4,6mCzP2Pm (3.18 eV). Thus, when the exciplex is formed, alight-emitting element with low driving voltage can be obtained.

The energy difference between the LUMO level of 4,4′mCzP2BPy and theHOMO level of Ir(ppz)₃ is 2.73 eV. This value is substantially equal tolight emission energy (2.44 eV) calculated from the peak wavelength ofthe electroluminescence spectrum of the light-emitting element 6 in FIG.36. This result implies that the electroluminescence spectrum of thelight-emitting element 6 corresponds to light emission due to theexciplex formed by 4,4′mCzP2BPy and Ir(ppz)₃. In the exciplex, thedifference between the S1 level and the T1 level is small; thus, thelight emission energy can be regarded as energy of each of the S1 leveland the T1 level (2.44 eV). Furthermore, the excitation energy level ofthe exciplex formed by 4,4′mCzP2BPy and Ir(ppz)₃ is smaller than theenergy difference between the LUMO level and the HOMO level of4,4′mCzP2BPy (3.28 eV). Thus, when the exciplex is formed, alight-emitting element with low driving voltage can be obtained.

Note that the compounds used in the light-emitting element 3 weresimilar to the compounds used in the light-emitting element 2 describedin Example 2. This means that the electroluminescence spectrum of thelight-emitting element 3 corresponds to light emission due to theexciplex formed by 4,6mCzP2Pm and Ir(ppz)₃. Furthermore, the lightemission energy can be regarded as energy of each of the S1 level andthe T1 level.

<Measurement of T1 Level>

Next, to obtain the T1 levels of the compounds each used in thelight-emitting layer 130, a thin film of 4,6mCzBP2Pm, a thin film of5Me-4,6mCzP2Pm, and a thin film of 4,4′mCzP2BPy were each formed over aquartz substrate by a vacuum evaporation method, and the emissionspectra of the thin films were measured at a low temperature (10 K).Note that the measurement method was similar to that used in Example 1.

FIG. 37, FIG. 38, and FIG. 39 show the time-resolved emission spectra of4,6mCzBP2Pm, 5Me-4,6mCzP2Pm, and 4,4′mCzP2BPy, respectively, measured ata low temperature.

As shown in the measurement results of the emission spectra, theemission spectrum of 4,6mCzBP2Pm has a peak (including a shoulder) ofthe phosphorescent component on the shortest wavelength side at 452 nm,the emission spectrum of 5Me-4,6mCzP2Pm has a peak (including ashoulder) of the phosphorescent component on the shortest wavelengthside at 446 nm, and the emission spectrum of 4,4′mCzP2BPy has a peak(including a shoulder) of the phosphorescent component on the shortestwavelength side at 447 nm.

Thus, from the peak wavelengths, the T1 level of 4,6mCzBP2Pm wascalculated to be 2.74 eV, the T1 level of 5Me-4,6mCzP2Pm was calculatedto be 2.78 eV, and the T1 level of 4,4′mCzP2BPy was calculated to be2.77 eV.

From the above measurement results, it is found that the T1 level of4,6mCzBP2Pm (2.74 eV) is lower than the T1 level of Ir(ppz)₃ (3.27 eV),higher than the T1 level of the exciplex (2.38 eV) formed by 4,6mCzBP2Pmand Ir(ppz)₃, and is greater than the difference between the LUMO levelof 4,6mCzBP2Pm and the HOMO level of Ir(ppz)₃ (2.59 eV). Thus, theexciplex formed by 4,6mCzBP2Pm and Ir(ppz)₃ can emit light efficiently.

Furthermore, it is found that the T1 level of 5Me-4,6mCzP2Pm (2.78 eV)is lower than the T1 level of Ir(ppz)₃ (3.27 eV), higher than the T1level of the exciplex (2.40 eV) formed by 5Me-4,6mCzP2Pm and Ir(ppz)₃,and is greater than the difference between the LUMO level of5Me-4,6mCzP2Pm and the HOMO level of Ir(ppz)₃ (2.66 eV). Thus, theexciplex formed by 5Me-4,6mCzP2Pm and Ir(ppz)₃ can emit lightefficiently.

Moreover, it is found that the T1 level of 4,4′mCzP2BPy (2.77 eV) islower than the T1 level of Ir(ppz)₃ (3.27 eV), higher than the T1 levelof the exciplex (2.44 eV) formed by 4,4′mCzP2BPy and Ir(ppz)₃, and isequivalent to the difference between the LUMO level of 4,4′mCzP2BPy andthe HOMO level of Ir(ppz)₃ (2.73 eV). Thus, the triplet excitationenergy of the exciplex formed by 4,4′mCzP2BPy and Ir(ppz)₃ is likely tobe transferred to the T1 level of 4,4′mCzP2BPy, and it is difficult forthe exciplex to emit light efficiently. Furthermore, as shown in FIG.36, the rising wavelength on the shorter wavelength side of theelectroluminescence spectrum of the light-emitting element 6 is shorterthan a wavelength (448 nm) that corresponds to the T1 level of4,4′mCzP2BPy (2.77 eV). Accordingly, to obtain efficient light emissionfrom an exciplex, it is preferable that the T1 level of each ofcompounds that form the exciplex be higher than energy obtained byconversion using the rising wavelength on the shorter wavelength side ofthe emission spectrum of the exciplex.

According to the above results, the difference between the T1 level ofone of the first organic compound and the second organic compound havinga lower T1 level and the T1 level of an exciplex formed by the firstorganic compound and the second organic compound is preferably large.Specifically, the T1 level of one of the first organic compound and thesecond organic compound having a lower T1 level is preferably higherthan the T1 level of the exciplex formed by the first organic compoundand the second organic compound by 0.35 eV or more. In addition, thedifference between the T1 level of one of the first organic compound andthe second organic compound having a lower T1 level and the differencebetween the lower LUMO level and the higher HOMO level of the first andsecond organic compounds is preferably large. Specifically, the T1 levelof one of the first organic compound and the second organic compoundhaving a lower T1 level is preferably greater than the differencebetween the lower LUMO level and the higher HOMO level of the first andsecond organic compounds by 0.1 eV or more.

When the measurement of emission spectrum of Ir(ppz)₃ was performed atroom temperature, light emission from Ir(ppz)₃ was not observed.Non-Patent Document 1 discloses that the luminescence quantum yield ofIr(ppz)₃ is lower than 1% at room temperature. This indicates thatIr(ppz)₃ is a material that does not emit light at room temperature.

Each of the light-emitting elements 3 to 6 is a light-emitting elementthat emits light originating from the exciplex formed by the secondorganic compound (4,6mCzP2Pm, 4,6mCzBP2Pm, 5Me-4,6mCzP2Pm, or4,4′mCzP2BPy) and the first organic compound (Ir(ppz)₃). This is becauseeach of the light-emitting elements 3 to 6 emits, in addition to lightoriginating from singlet excitons generated by recombination of carriers(holes and electrons) injected from the pair of electrodes, lightoriginating from triplet excitons or light originating from singletexcitons generated from triplet excitons by reverse intersystem crossingin the exciplex. That is, even when a compound with a low luminescencequantum yield, which is lower than 1%, is used, one embodiment of thepresent invention can provide a light-emitting element with highluminous efficiency.

With one embodiment of the present invention, a light-emitting elementwith high luminous efficiency can be provided. With one embodiment ofthe present invention, a light-emitting element with low driving voltageand low power consumption can be provided.

Example 4

In this example, examples of fabrication methods of the light-emittingelements of one embodiment of the present invention will be described.The structure of each of the light-emitting elements fabricated in thisexample is the same as that illustrated in FIG. 1. Table 7 shows thedetails of the element structures. In addition, structures andabbreviations of compounds used here are given below. Note that Example2 is referred to for structures and abbreviations of other compounds.

TABLE 7 Reference Thickness Weight Layer numeral (nm) Material ratioLight-emitting Electrode 102 200 Al — element 7 Electron-injection 119 1LiF — layer Electron-transport 118 (2) 10 BPhen — layer 118 (1) 204,6mCzP2Pm — Light-emitting layer 130 40 4,6mCzP2Pm:fac-Ir(pmb)₃ 1:0.1Hole-transport layer 112 20 mCzFLP — Hole-injection layer 111 60DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — Light-emitting Electrode 102200 Al — element 8 Electron-injection 119 1 LiF — layerElectron-transport 118 (2) 10 BPhen — layer 118 (1) 20 4,6mCzP2Pm —Light-emitting layer 130 40 4,6mCzP2Pm:mer-Ir(pmb)₃ 1:0.1 Hole-transportlayer 112 20 mCzFLP — Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5Electrode 101 70 ITSO —<Fabrication of Light-Emitting Elements 7 and 8>

Methods for fabricating the light-emitting elements 7 and 8 fabricatedin this example will be described below. The light-emitting elements 7and 8 were fabricated through the same steps as those for theabove-described light-emitting element 1 except for the step of formingthe light-emitting layer 130.

As the light-emitting layer 130 of the light-emitting element 7,4,6mCzP2Pm andfac-tris(1-phenyl-3-methylbenzimidazolin-2-ylidene-C,C²′)iridium(III)(abbreviation: fac-Ir(pmb)₃) were deposited to a thickness of 40 nm byco-evaporation at a weight ratio of 1:0.1 (4,6mCzP2Pm:fac-Ir(pmb)₃). Inthe light-emitting layer 130, fac-Ir(pmb)₃ corresponded to the firstorganic compound, and 4,6mCzP2Pm corresponded to the second organiccompound.

As the light-emitting layer 130 of the light-emitting element 8,4,6mCzP2Pm andmer-tris(1-phenyl-3-methylbenzimidazolin-2-ylidene-C,C²′)iridium(III)(abbreviation: mer-Ir(pmb)₃) were deposited to a thickness of 40 nm byco-evaporation at a weight ratio of 1:0.1 (4,6mCzP2Pm:mer-Ir(pmb)₃). Inthe light-emitting layer 130, mer-Ir(pmb)₃ corresponded to the firstorganic compound, and 4,6mCzP2Pm corresponded to the second organiccompound.

<Characteristics of Light-Emitting Elements>

Next, the characteristics of the fabricated light-emitting elements 7and 8 were measured. Note that the measurement method was similar tothat used in Example 1.

FIG. 40 shows the luminance-current density characteristics of thelight-emitting elements 7 and 8. FIG. 41 shows the luminance-voltagecharacteristics thereof. FIG. 42 shows the current efficiency-luminancecharacteristics thereof. FIG. 43 shows the power efficiency-luminancecharacteristics thereof. FIG. 44 shows the external quantumefficiency-luminance characteristics thereof. FIG. 45 shows theelectroluminescence spectra obtained when a current at a current densityof 2.5 mA/cm² was supplied to the light-emitting elements 7 and 8. Themeasurements of the light-emitting elements were performed at roomtemperature (in an atmosphere kept at 23° C.).

Table 8 shows the element characteristics of the light-emitting elements7 and 8 at around 1000 cd/m².

TABLE 8 External Current CIE Current Power quantum Voltage densitychromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 4.20 3.91 (0.350, 0.559) 89022.8 17.1 7.23 element 7 Light-emitting 4.00 3.32 (0.405, 0.550) 104031.4 24.6 10.1 element 8

As shown in FIG. 45, the electroluminescence spectra of thelight-emitting elements 7 and 8 have peaks at wavelengths of 536 nm and551 nm, respectively, and have large full widths at half maximum of 106nm and 109 nm, respectively. This indicates that the light-emittingelements 7 and 8 emit yellow light. Note that although fac-Ir(pmb)₃ andmer-Ir(pmb)₃ used for the light-emitting element 7 and thelight-emitting element 8 were compounds that emit blue light asdescribed later, no light emission originating from fac-Ir(pmb)₃ ormer-Ir(pmb)₃ was observed from the light-emitting elements 7 and 8.

As shown in FIG. 40 to FIG. 44 and Table 8, the driving voltage of eachof the light-emitting elements 7 and 8 is low. Furthermore, thelight-emitting elements 7 and 8 have high luminous efficiency (currentefficiency, power efficiency, and external quantum efficiency).Accordingly, the light-emitting element of one embodiment of the presentinvention that contains the first organic compound (fac-Ir(pmb)₃ ormer-Ir(pmb)₃) and the second organic compound (4,6mCzP2Pm) is alight-emitting element with high luminous efficiency that drives at lowdriving voltage at low power consumption.

<CV Measurement Results>

The electrochemical characteristics (oxidation reaction characteristicsand reduction reaction characteristics) of the above compounds weremeasured by cyclic voltammetry (CV) measurement. Note that themeasurement method was similar to that used in Example 1.

According to the CV measurement results, the oxidation potential offac-Ir(pmb)₃ was 0.55 V. The HOMO level of fac-Ir(pmb)₃ calculated fromthe CV measurement results was −5.49 eV. Thus, it is found thatfac-Ir(pmb)₃ has a high HOMO level. The oxidation potential ofmer-Ir(pmb)₃ was 0.38 V. The HOMO level of mer-Ir(pmb)₃ calculated fromthe CV measurement results was −5.32 eV. Thus, it is found thatmer-Ir(pmb)₃ has a high HOMO level. Note that the LUMO levels offac-Ir(pmb)₃ and mer-Ir(pmb)₃ are probably high because the reductionpotentials of fac-Ir(pmb)₃ and mer-Ir(pmb)₃ were low and no clearreduction peak was observed.

As described above, the LUMO level of 4,6mCzP2Pm, which is the secondorganic compound, is lower than the LUMO level of each of fac-Ir(pmb)₃and mer-Ir(pmb)₃, which are the first organic compounds. Furthermore,the HOMO level of 4,6mCzP2Pm, which is the second organic compound, islower than the HOMO level of each of fac-Ir(pmb)₃ and mer-Ir(pmb)₃,which are the first organic compounds. Thus, in the case where thecompounds are used in a light-emitting layer as in the light-emittingelements 7 and 8, electrons and holes, which serve as carriers, areefficiently injected from a pair of electrodes into the second organiccompound (4,6mCzP2Pm) and the first organic compound (fac-Ir(pmb)₃ ormer-Ir(pmb)₃), respectively, so that the second organic compound(4,6mCzP2Pm) and the first organic compound (fac-Ir(pmb)₃ ormer-Ir(pmb)₃) can form an exciplex.

The exciplex formed by the second organic compound (4,6mCzP2Pm) and thefirst organic compound (fac-Ir(pmb)₃ or mer-Ir(pmb)₃) has the LUMO levelin the second organic compound (4,6mCzP2Pm) and the HOMO level in thefirst organic compound (fac-Ir(pmb)₃ or mer-Ir(pmb)₃).

The energy difference between the LUMO level of 4,6mCzP2Pm and the HOMOlevel of fac-Ir(pmb)₃ is 2.61 eV. This value is substantially equal tolight emission energy (2.31 eV) calculated from the peak wavelength ofthe electroluminescence spectrum of the light-emitting element 7 in FIG.45. This result implies that the electroluminescence spectrum of thelight-emitting element 7 corresponds to light emission due to theexciplex formed by 4,6mCzP2Pm and fac-Ir(pmb)₃. In the exciplex, thedifference between the S1 level and the T1 level is small; thus, thelight emission energy can be regarded as energy of each of the S1 leveland the T1 level (2.31 eV). Furthermore, the excitation energy level ofthe exciplex formed by 4,6mCzP2Pm and fac-Ir(pmb)₃ is smaller than theenergy difference between the LUMO level and the HOMO level of4,6mCzP2Pm (i.e., 3.01 eV). Thus, when the exciplex is formed, alight-emitting element with low driving voltage can be obtained.

The energy difference between the LUMO level of 4,6mCzP2Pm and the HOMOlevel of mer-Ir(pmb)₃ is 2.44 eV. This value is substantially equal tolight emission energy (2.25 eV) calculated from the peak wavelength ofthe electroluminescence spectrum of the light-emitting element 8 in FIG.45. This result implies that the electroluminescence spectrum of thelight-emitting element 8 corresponds to light emission due to theexciplex formed by 4,6mCzP2Pm and mer-Ir(pmb)₃. In the exciplex, thedifference between the S1 level and the T1 level is small; thus, thelight emission energy can be regarded as energy of each of the S1 leveland the T1 level (2.25 eV). Furthermore, the excitation energy level ofthe exciplex formed by 4,6mCzP2Pm and mer-Ir(pmb)₃ is smaller than theenergy difference between the LUMO level and the HOMO level of4,6mCzP2Pm (i.e., 3.01 eV). Thus, when the exciplex is formed, alight-emitting element with low driving voltage can be obtained.

<Absorption Spectrum and Emission Spectrum of Compound>

Next, FIGS. 46A and 46B show the measurement results of the absorptionand emission spectra of fac-Ir(pmb)₃ and mer-Ir(pmb)₃.

For the measurement of the absorption and emission spectra, adichloromethane solution in which fac-Ir(pmb)₃ or mer-Ir(pmb)₃ wasdissolved was prepared, and a quartz cell was used. The absorptionspectrum was measured using an ultraviolet-visible spectrophotometer(V-550, produced by JASCO Corporation). The absorption spectra of thequartz cell and dichloromethane were subtracted from the measuredspectrum of the solution. Note that the emission spectrum of thesolution was measured with a PL-EL measurement apparatus (produced byHamamatsu Photonics K.K.). The measurement was performed at roomtemperature (in an atmosphere kept at 23° C.).

As shown in FIGS. 46A and 46B, an absorption edge on the lowest energyside (the longest wavelength side) of the absorption spectrum of each offac-Ir(pmb)₃ and mer-Ir(pmb)₃ is at around 380 nm. The absorption edgeswere obtained from data of the absorption spectra, and transitionenergies were estimated on the assumption of direct transition, wherebytransition energies of fac-Ir(pmb)₃ and mer-Ir(pmb)₃ were calculated tobe 3.20 eV and 3.15 eV, respectively. Since fac-Ir(pmb)₃ andmer-Ir(pmb)₃ are each a phosphorescent compound, the absorption band onthe lowest energy side is based on the transition to the triplet excitedstate. Therefore, the T1 levels of fac-Ir(pmb)₃ and mer-Ir(pmb)₃ arecalculated to be 3.20 eV and 3.15 eV, respectively.

From the above measurement results, it is found that the T1 level of4,6mCzP2Pm (2.70 eV) is lower than the T1 level of fac-Ir(pmb)₃ (3.20eV), higher than the T1 level of the exciplex (2.31 eV) formed by4,6mCzP2Pm and fac-Ir(pmb)₃, and is greater than the difference betweenthe LUMO level of 4,6mCzP2Pm and the HOMO level of fac-Ir(pmb)₃ (2.61eV). Thus, the exciplex formed by 4,6mCzP2Pm and fac-Ir(pmb)₃ can emitlight efficiently.

Furthermore, it is found that the T1 level of 4,6mCzP2Pm (2.70 eV) islower than the T1 level of mer-Ir(pmb)₃ (3.15 eV), higher than the T1level of the exciplex (2.25 eV) formed by 4,6mCzP2Pm and mer-Ir(pmb)₃,and is greater than the difference between the LUMO level of 4,6mCzP2Pmand the HOMO level of mer-Ir(pmb)₃ (2.44 eV). Thus, the exciplex formedby 4,6mCzP2Pm and mer-Ir(pmb)₃ can emit light efficiently.

Moreover, the light-emitting element 8 was driven with higher efficiencyand lower driving voltage than the light-emitting element 7. Accordingto the above, the energy difference between either of the T1 levels ofthe first and second organic compounds, whichever is lower, and the T1level of an exciplex formed by the first and second organic compounds ispreferably large. Specifically, either of the T1 levels of the first andsecond organic compounds, whichever is lower, is preferably higher thanthe T1 level of the exciplex formed by the first and second organiccompounds by 0.35 eV or more. In addition, the energy difference betweeneither of the T1 levels of the first and second organic compounds,whichever is lower, and the difference between the lower LUMO level andthe higher HOMO level of the first and second organic compounds ispreferably large. Specifically, either of the T1 levels of the first andsecond organic compounds, whichever is lower, is preferably greater thanthe difference between the lower LUMO level and the higher HOMO level ofthe first and second organic compounds by 0.1 eV or more.

Non-Patent Document 1 discloses that the luminescence quantum yield offac-Ir(pmb)₃ is 37% at room temperature. Thus, it is found thatfac-Ir(pmb)₃ is a light-emitting material with a low luminescencequantum yield.

Each of the light-emitting elements 7 and 8 is a light-emitting elementthat emits light originating from the exciplex formed by the secondorganic compound (4,6mCzP2Pm) and the first organic compound(fac-Ir(pmb)₃ or mer-Ir(pmb)₃). The light-emitting elements 7 and 8 canhave favorable element characteristics because each of thelight-emitting elements 7 and 8 emits, in addition to light originatingfrom singlet excitons generated by recombination of carriers (holes andelectrons) injected from the pair of electrodes, light originating fromtriplet excitons or light originating from singlet excitons generatedfrom triplet excitons by reverse intersystem crossing in the exciplex.That is, even when a compound with a low luminescence quantum yield isused, one embodiment of the present invention can provide alight-emitting element with high luminous efficiency.

With one embodiment of the present invention, a light-emitting elementwith high luminous efficiency can be provided. With one embodiment ofthe present invention, a light-emitting element with low driving voltageand low power consumption can be provided.

Example 5

In this example, examples of fabrication methods of the light-emittingelement 3, which was the light-emitting element of one embodiment of thepresent invention, and a comparative light-emitting element 9 will bedescribed. In the comparative light-emitting element 9, host materialsthat form an exciplex in a light-emitting layer and do not has afunction of converting triplet excitation energy into light emissionwere used. The structure of each of the light-emitting elementsfabricated in this example is the same as that illustrated in FIG. 1.Table 9 shows the details of the element structures. In addition,structures and abbreviations of compounds used here are given below.Note that Examples 1 to 4 are referred to for structures andabbreviations of other compounds.

TABLE 9 Reference Thickness Weight Layer numeral (nm) Material ratioLight-emitting Electrode 102 200 Al — element 3 Electron-injection 119 1LiF — layer Electron-transport 118 (2) 10 BPhen — layer 118 (1) 204,6mCzP2Pm — Light-emitting layer 130 40 4,6mCzP2Pm:Ir(ppz)₃ 1:0.1Hole-transport layer 112 20 mCzFLP — Hole-injection layer 111 60DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — Comparative Electrode 102200 Al — light-emitting Electron-injection 119 1 LiF — element 9 layerElectron-transport 118 (2) 15 NBPhen — layer 118 (1) 20 4,6mCzP2Pm —Light-emitting layer 130 40 4,6mCzP2Pm:PCBiF 0.8:0.2   Hole-transportlayer 112 20 BPAFLP — Hole-injection layer 111 40 DBT3P-II:MoO₃ 1:0.5Electrode 101 70 ITSO —<Fabrication of Comparative Light-Emitting Element 9>

As the electrode 101, an ITSO film was formed to a thickness of 70 nmover a glass substrate. The electrode area of the electrode 101 was setto 4 mm² (2 mm×2 mm).

As the hole-injection layer 111, DBT3P-II and molybdenum oxide (MoO₃)were deposited to a thickness of 40 nm over the electrode 101 byco-evaporation at a weight ratio of 1:0.5 (DBT3P-II:MoO₃).

As the hole-transport layer 112,4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP)was deposited over the hole-injection layer 111 by evaporation to athickness of 20 nm.

As the light-emitting layer 130, 4,6mCzP2Pm andN-(4-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carbazol-3-amine(abbreviation: PCBiF) were deposited to a thickness of 40 nm over thehole-transport layer 112 by co-evaporation at a weight ratio of 0.8:0.2(4,6mCzP2Pm:PCBiF).

As the electron-transport layer 118, 4,6mCzP2Pm and2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation:NBPhen) were sequentially deposited by evaporation to thicknesses of 20nm and 15 nm, respectively, over the light-emitting layer 130. Then, asthe electron-injection layer 119, LiF was deposited over theelectron-transport layer 118 by evaporation to a thickness of 1 nm.

As the electrode 102, aluminum (Al) was formed over theelectron-injection layer 119 to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, the electrodesand the EL layer were sealed by fixing a glass substrate for sealing tothe glass substrate on which the organic materials were deposited usinga sealant for an organic EL device. Specifically, after the sealant wasapplied so as to surround the organic materials deposited on the glasssubstrate and the glass substrate was bonded to the glass substrate forsealing, irradiation with ultraviolet light having a wavelength of 365nm at 6 J/cm² and heat treatment at 80° C. for one hour were performed.Through the above process, the comparative light-emitting element 9 wasfabricated.

<Characteristics of Light-Emitting Elements>

Next, the characteristics of the fabricated light-emitting element 3 andcomparative light-emitting element 9 were measured. Note that themeasurement method was similar to that used in Example 1.

FIG. 47 shows the external quantum efficiency-luminance characteristicsof the light-emitting element 3 and the comparative light-emittingelement 9. FIG. 48 shows the electroluminescence spectra obtained when acurrent at a current density of 2.5 mA/cm² was supplied to thelight-emitting element 3 and the comparative light-emitting element 9.The measurements of the light-emitting elements were performed at roomtemperature (in an atmosphere kept at 23° C.).

Table 10 shows the element characteristics of the light-emitting element3 and the comparative light-emitting element 9 at around 1000 cd/m².

TABLE 10 External Current CIE Current Power quantum Voltage densitychromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 3.60 1.57 (0.327, 0.580) 97061.3 53.5 18.6 element 3 Comparative light- 3.60 2.05 (0.402, 0.570) 91044.5 38.8 13.2 emitting element 9

As shown in FIG. 48, the light-emitting element 3 and the comparativelight-emitting element 9 have peaks at wavelengths of 526 nm and 548 nm,respectively, and have large full widths at half maximum are 95 nm and91 nm, respectively. This indicates that the light-emitting element 3and the comparative light-emitting element 9 emit yellow light. Asdescribed above, light emission obtained from the light-emitting element3 is light emission from the exciplex formed by 4,6mCzP2Pm and Ir(ppz)₃.

The light emission energy calculated from the peak wavelength of lightemission obtained from the comparative light-emitting element 9 is 2.26eV. This value is substantially equal to the energy difference (2.38 eV)between the LUMO level of 4,6mCzP2Pm (−2.88 eV) and the HOMO level ofPCBiF (−5.26 eV), which is calculated by the CV measurement described inExample 1. That is, it can be said that light emission obtained from thecomparative light-emitting element 9 is light emission originating fromthe exciplex formed by 4,6mCzP2Pm and PCBiF in the light-emitting layer.

FIG. 47 and Table 10 show that the light-emitting element 3 has higherexternal quantum efficiency than the comparative light-emitting element9. This is because a compound containing a heavy atom was used as one ofcompounds that form an exciplex in the light-emitting element 3. Withone embodiment of the present invention, a light-emitting element withhigh luminous efficiency can be provided.

<Quantum Yield Measurement of Thin Film>

Next, the quantum yield of a mixed film of 4,6mCzP2Pm and Ir(ppz)₃ usedin the light-emitting layer of the light-emitting element 3 wasmeasured. To fabricate a measurement sample, 4,6mCzP2Pm and Ir(ppz)₃were deposited by evaporation to have a thickness of 40 nm and a weightratio (4,6mCzP2Pm:Ir(ppz)₃) of 1:0.1 over a quartz substrate and weresealed in a glove box containing a nitrogen atmosphere so as not to beexposed to the air. The measurement was performed with the use of anabsolute PL quantum yield measurement system (C11347-01 produced byHamamatsu Photonics K.K.) at room temperature.

The obtained quantum yield of the mixed film of 4,6mCzP2Pm and Ir(ppz)₃was 64%. Assuming that the light extraction efficiency of glass isapproximately 30%, the external quantum efficiency of the light-emittingelement 3 is estimated to be approximately 19%. As shown in FIG. 47, themaximum external quantum efficiency of the light-emitting element 3 is19.7%, which is substantially the same as the estimated value obtainedfrom the measurement results of the quantum yield of the mixed film of4,6mCzP2Pm and Ir(ppz)₃. This indicates that the loss of excitonsgenerated in the light-emitting element is mostly due to the quantumefficiency of the mixed film of 4,6mCzP2Pm and Ir(ppz)₃. Accordingly,not only singlet excitons but also triplet excitons, that is, allgenerated excitons are considered to contribute to light emission. Thisis probably because a compound containing a heavy atom was used as oneof the compounds that form an exciplex.

<Transient EL Measurement of Thin Films>

Next, the light-emitting element 3 and the comparative light-emittingelement 9 were subjected to transient EL measurement. A picosecondfluorescence lifetime measurement system (manufactured by HamamatsuPhotonics K.K.) was used for the measurement. To measure the lifetimesof fluorescence in the light-emitting elements, a square wave pulsevoltage was applied to the light-emitting elements, and time-resolvedmeasurement of light, which was attenuated from the falling of thevoltage, was performed using a streak camera. The pulse voltage wasapplied at a frequency of 10 Hz. By integrating data obtained byrepeated measurement, data with a high S/N ratio was obtained. Themeasurement was performed at room temperature (300 K) under theconditions of a pulse voltage of approximately 3 V, a pulse time widthof 100 μsec, a negative bias voltage of −5 V, and a measurement time of20 μsec for the light-emitting element 3 and 50 μsec for the comparativelight-emitting element 9. FIG. 49 shows the results.

As shown in FIG. 49, the comparative light-emitting element 9 contains alarge proportion of delayed fluorescent components and the lightemission lifetime is extremely long. In contrast, the proportion ofdelayed fluorescent components contained in the light-emitting element 3is not large; however, the external quantum efficiency of thelight-emitting element 3 is higher than that of the comparativelight-emitting element 9 as shown in FIG. 47. In addition, as shown inFIG. 49, the light emission lifetime of the light-emitting element 3 isextremely short. This indicates that the excited state is deactivated tothe ground state in a short time. Such a light-emitting element with ashort light emission lifetime is highly reliable, which is preferable.

It is probable that, since the light-emitting element 3 contains Ir,which is a heavy atom, in the material used for the exciplex, thebehavior of the exciplex in the light-emitting element 3 is differentfrom those of the exciplex formed using normal host materials and theTADF material.

With one embodiment of the present invention, a light-emitting elementwith high luminous efficiency can be provided. In addition, with oneembodiment of the present invention, a highly reliable light-emittingelement can be provided.

This application is based on Japanese Patent Application serial no.2016-093149 filed with Japan Patent Office on May 6, 2016, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting element comprising alight-emitting layer comprising a first organic compound and a secondorganic compound, wherein a LUMO level of one of the first organiccompound and the second organic compound is higher than or equal to aLUMO level of the other of the first organic compound and the secondorganic compound, wherein a HOMO level of the one of the first organiccompound and the second organic compound is higher than or equal to aHOMO level of the other of the first organic compound and the secondorganic compound, wherein a combination of the first organic compoundand the second organic compound is capable of forming an exciplex,wherein the first organic compound is capable of converting tripletexcitation energy into light emission, wherein the first organiccompound has a luminescence quantum yield higher than or equal to 0% andlower than or equal to 40% at room temperature, and wherein lightemitted from the light-emitting layer comprises light emitted from theexciplex.
 2. The light-emitting element according to claim 1, wherein alowest triplet excitation energy level of the first organic compound ishigher than or equal to a lowest triplet excitation energy level of thesecond organic compound.
 3. The light-emitting element according toclaim 1, wherein the first organic compound comprises a ligandcoordinated to Ir.
 4. The light-emitting element according to claim 3,wherein the ligand comprises a nitrogen-containing five-memberedheterocyclic skeleton.
 5. The light-emitting element according to claim1, wherein the second organic compound is capable of transporting anelectron, and wherein the second organic compound comprises a π-electrondeficient heteroaromatic skeleton.
 6. The light-emitting elementaccording to claim 1, wherein the exciplex is capable of emitting lightwith luminous efficiency higher than luminous efficiency of lightemitted from the first organic compound.
 7. A display device comprising:the light-emitting element according to claim 1; and at least one of acolor filter and a transistor.
 8. A light-emitting element comprising alight-emitting layer comprising a first organic compound and a secondorganic compound, wherein a LUMO level of one of the first organiccompound and the second organic compound is higher than or equal to aLUMO level of the other of the first organic compound and the secondorganic compound, wherein a HOMO level of the one of the first organiccompound and the second organic compound is higher than or equal to aHOMO level of the other of the first organic compound and the secondorganic compound, wherein a combination of the first organic compoundand the second organic compound is capable of forming an exciplex,wherein the first organic compound is capable of not emittingfluorescence, wherein the first organic compound is capable of emittingphosphorescence, wherein the first organic compound has a luminescencequantum yield higher than or equal to 0% and lower than or equal to 40%at room temperature, and wherein light emitted from the light-emittinglayer comprises light emitted from the exciplex.
 9. The light-emittingelement according to claim 8, wherein a lowest triplet excitation energylevel of the first organic compound is higher than or equal to a lowesttriplet excitation energy level of the second organic compound.
 10. Thelight-emitting element according to claim 8, wherein the first organiccompound comprises a ligand coordinated to Ir.
 11. The light-emittingelement according to claim 10, wherein the ligand comprises anitrogen-containing five-membered heterocyclic skeleton.
 12. Thelight-emitting element according to claim 8, wherein the second organiccompound is capable of transporting an electron, and wherein the secondorganic compound comprises a π-electron deficient heteroaromaticskeleton.
 13. The light-emitting element according to claim 8, whereinthe exciplex is capable of emitting light with luminous efficiencyhigher than luminous efficiency of light emitted from the first organiccompound.
 14. A display device comprising: the light-emitting elementaccording to claim 8; and at least one of a color filter and atransistor.
 15. A light-emitting element comprising a light-emittinglayer comprising a first organic compound and a second organic compound,wherein a LUMO level of one of the first organic compound and the secondorganic compound is higher than or equal to a LUMO level of the other ofthe first organic compound and the second organic compound, wherein aHOMO level of the one of the first organic compound and the secondorganic compound is higher than or equal to a HOMO level of the other ofthe first organic compound and the second organic compound, wherein acombination of the first organic compound and the second organiccompound is capable of forming an exciplex, wherein the first organiccompound comprises Ru, Rh, Pd, Os, Ir, or Pt, wherein the first organiccompound has a luminescence quantum yield higher than or equal to 0% andlower than or equal to 40% at room temperature, and wherein lightemitted from the light-emitting layer comprises light emitted from theexciplex.
 16. The light-emitting element according to claim 15, whereina lowest triplet excitation energy level of the first organic compoundis higher than or equal to a lowest triplet excitation energy level ofthe second organic compound.
 17. The light-emitting element according toclaim 15, wherein the first organic compound comprises a ligandcoordinated to Ir.
 18. The light-emitting element according to claim 17,wherein the ligand comprises a nitrogen-containing five-memberedheterocyclic skeleton.
 19. The light-emitting element according to claim15, wherein the second organic compound is capable of transporting anelectron, and wherein the second organic compound comprises a π-electrondeficient heteroaromatic skeleton.
 20. The light-emitting elementaccording to claim 15, wherein the exciplex is capable of emitting lightwith luminous efficiency higher than luminous efficiency of lightemitted from the first organic compound.
 21. A display devicecomprising: the light-emitting element according to claim 15; and atleast one of a color filter and a transistor.